The bug is easy to reproduce:
1. install the coverflow extension
2. enable it in banshee
3. restart banshee

Then you will get the following error messages:
[Info  21:27:06.287] Running Banshee 2.0.0: [Ubuntu Natty (development branch) (linux-gnu, i686) @ 2011-04-18 16:21:33 UTC]
[Info  21:27:07.326] Updating web proxy from GConf
failed to create drawable

(Banshee:13194): Clutter-CRITICAL **: Unable to initialize Clutter: Unable to select the newly created GLX context
[Warn  21:27:07.370] Caught an exception - System.NotSupportedException: Unable to initialize GtkClutter (in `Banshee.ClutterFlow')
  at Banshee.ClutterFlow.ClutterFlowManager.Init () [0x00000] in <filename unknown>:0 
  at Banshee.ClutterFlow.ClutterFlowService..ctor () [0x00000] in <filename unknown>:0 
  at (wrapper managed-to-native) System.Reflection.MonoCMethod:InternalInvoke (object,object[],System.Exception&)
  at System.Reflection.MonoCMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00000] in <filename unknown>:0 
[Warn  21:27:07.370] Extension `/Banshee/ServiceManager/Service/__nid_13' not started: Exception has been thrown by the target of an invocation.
**
Cogl-glx:ERROR:./winsys/cogl-xlib.c:87:_cogl_xlib_set_display: assertion failed: (_cogl_xlib_display == NULL)
Stacktrace:

  at (wrapper managed-to-native) ClutterFlow.ClutterHelper.gtk_clutter_init (intptr,intptr) <0x00004>
  at (wrapper managed-to-native) ClutterFlow.ClutterHelper.gtk_clutter_init (intptr,intptr) <0x00004>
  at Banshee.ClutterFlow.ClutterFlowManager.Init () <0x00035>
  at Banshee.ClutterFlow.ClutterFlowService..ctor () <0x0000a>
  at (wrapper runtime-invoke) object.runtime_invoke_void__this__ (object,intptr,intptr,intptr) <0x00040>
  at (wrapper managed-to-native) System.Reflection.MonoCMethod.InternalInvoke (object,object[],System.Exception&) <0x00004>
  at (wrapper managed-to-native) System.Reflection.MonoCMethod.InternalInvoke (object,object[],System.Exception&) <0x00004>
  at System.Reflection.MonoCMethod.Invoke (object,System.Reflection.BindingFlags,System.Reflection.Binder,object[],System.Globalization.CultureInfo) <0x0018b>
  at System.Reflection.MonoCMethod.Invoke (System.Reflection.BindingFlags,System.Reflection.Binder,object[],System.Globalization.CultureInfo) <0x00027>
  at System.Reflection.ConstructorInfo.Invoke (object[]) <0x00042>
  at System.Activator.CreateInstance (System.Type,bool) <0x00184>
  at System.Activator.CreateInstance (System.Type) <0x00012>
  at Mono.Addins.TypeExtensionNode.CreateInstance () <0x0001c>
  at Mono.Addins.InstanceExtensionNode.CreateInstance (System.Type) <0x00015>
  at Banshee.ServiceStack.ServiceManager.StartExtension (Mono.Addins.TypeExtensionNode) <0x0006e>
  at Banshee.ServiceStack.ServiceManager.OnExtensionChanged (object,Mono.Addins.ExtensionNodeEventArgs) <0x0007b>
  at Mono.Addins.ExtensionNode.add_ExtensionNodeChanged (Mono.Addins.ExtensionNodeEventHandler) <0x00109>
  at Mono.Addins.ExtensionContext.AddExtensionNodeHandler (string,Mono.Addins.ExtensionNodeEventHandler) <0x0002e>
  at Mono.Addins.AddinManager.AddExtensionNodeHandler (string,Mono.Addins.ExtensionNodeEventHandler) <0x00027>
  at Banshee.ServiceStack.ServiceManager.Run () <0x002d3>
  at Banshee.ServiceStack.Application.Run () <0x00046>
  at Banshee.Gui.GtkBaseClient.Initialize (bool) <0x0027e>
  at Banshee.Gui.GtkBaseClient..ctor (bool,string) <0x00023>
  at Banshee.Gui.GtkBaseClient..ctor () <0x00017>
  at Nereid.Client..ctor () <0x00010>
  at (wrapper runtime-invoke) object.runtime_invoke_void__this__ (object,intptr,intptr,intptr) <0x00040>
  at (wrapper managed-to-native) System.Reflection.MonoCMethod.InternalInvoke (object,object[],System.Exception&) <0x00004>
  at (wrapper managed-to-native) System.Reflection.MonoCMethod.InternalInvoke (object,object[],System.Exception&) <0x00004>
  at System.Reflection.MonoCMethod.Invoke (object,System.Reflection.BindingFlags,System.Reflection.Binder,object[],System.Globalization.CultureInfo) <0x0018b>
  at System.Reflection.MonoCMethod.Invoke (System.Reflection.BindingFlags,System.Reflection.Binder,object[],System.Globalization.CultureInfo) <0x00027>
  at System.Reflection.ConstructorInfo.Invoke (object[]) <0x00042>
  at System.Activator.CreateInstance (System.Type,bool) <0x00184>
  at System.Activator.CreateInstance (System.Type) <0x00012>
  at Banshee.Gui.GtkBaseClient.Startup () <0x00015>
  at Hyena.Gui.CleanRoomStartup.Startup (Hyena.Gui.CleanRoomStartup/StartupInvocationHandler) <0x00089>
  at Banshee.Gui.GtkBaseClient.Startup<object> () <0x0005c>
  at Banshee.Gui.GtkBaseClient.Startup<object> (string[]) <0x000d8>
  at Nereid.Client.Main (string[]) <0x00015>
  at (wrapper runtime-invoke) <Module>.runtime_invoke_void_object (object,intptr,intptr,intptr) <0x00043>
  at (wrapper managed-to-native) System.AppDomain.ExecuteAssembly (System.Reflection.Assembly,string[]) <0x00004>
  at (wrapper managed-to-native) System.AppDomain.ExecuteAssembly (System.Reflection.Assembly,string[]) <0x00004>
  at System.AppDomain.ExecuteAssemblyInternal (System.Reflection.Assembly,string[]) <0x0002e>
  at System.AppDomain.ExecuteAssembly (string,System.Security.Policy.Evidence,string[]) <0x00025>
  at (wrapper remoting-invoke-with-check) System.AppDomain.ExecuteAssembly (string,System.Security.Policy.Evidence,string[]) <0x00067>
  at System.AppDomain.ExecuteAssembly (string) <0x00019>
  at (wrapper remoting-invoke-with-check) System.AppDomain.ExecuteAssembly (string) <0x00057>
  at Booter.Booter.BootClient (string) <0x00069>
  at Booter.Booter.Main () <0x001a0>
  at (wrapper runtime-invoke) object.runtime_invoke_void (object,intptr,intptr,intptr) <0x0003a>

Native stacktrace:

	banshee() [0x80dbc5b]
	[0xb781640c]
	/lib/i386-linux-gnu/libc.so.6(abort+0x17e) [0xb75a734e]
	/lib/i386-linux-gnu/libglib-2.0.so.0(g_assertion_message+0x150) [0xb778f3a0]
	/lib/i386-linux-gnu/libglib-2.0.so.0(+0x6897d) [0xb778f97d]
	/usr/lib/libclutter-glx-1.0.so.0(+0xfe550) [0xab55b550]
	/usr/lib/libclutter-glx-1.0.so.0(+0x265f3) [0xab4835f3]
	/usr/lib/libclutter-glx-1.0.so.0(+0x2fb15) [0xab48cb15]
	/usr/lib/libclutter-glx-1.0.so.0(+0x51150) [0xab4ae150]
	/usr/lib/libclutter-glx-1.0.so.0(+0x7a8ec) [0xab4d78ec]
	/usr/lib/libclutter-glx-1.0.so.0(+0x7ab92) [0xab4d7b92]
	/lib/i386-linux-gnu/libglib-2.0.so.0(g_option_context_parse+0x402) [0xb7775532]
	/usr/lib/libclutter-glx-1.0.so.0(clutter_init+0xd2) [0xab4d8fb2]
	/usr/lib/libclutter-gtk-0.10.so.0(gtk_clutter_init+0x5d) [0xab3bda9d]
	[0xab61e908]
	[0xab61e7ae]
	[0xab61e74b]
	[0xb5747329]
	banshee() [0x8062bc8]
	banshee(mono_runtime_invoke+0x3e) [0x8192eee]
	banshee(mono_runtime_invoke_array+0x5c8) [0x8196ac8]
	banshee() [0x814a098]
	[0xb5747250]
	[0xb5746e8c]
	[0xb5746cf8]
	[0xb5746cb3]
	[0xb57459bd]
	[0xb5745823]
	[0xb3e8fc65]
	[0xb3e8fbd6]
	[0xab74ce97]
	[0xab61fbfc]
	[0xb349cc4a]
	[0xb349ca77]
	[0xb349ca30]
	[0xb3e98984]
	[0xb3e954d7]
	[0xb4f27957]
	[0xb4f2764c]
	[0xb4f27610]
	[0xb4f275e1]
	[0xb5747329]
	banshee() [0x8062bc8]
	banshee(mono_runtime_invoke+0x3e) [0x8192eee]
	banshee(mono_runtime_invoke_array+0x5c8) [0x8196ac8]
	banshee() [0x814a098]
	[0xb5747250]
	[0xb5746e8c]
	[0xb5746cf8]
	[0xb5746cb3]
	[0xb57459bd]
	[0xb5745823]
	[0xb4f2758e]
	[0xb4f2745a]
	[0xb4f2738d]
	[0xb4f6c269]
	[0xb4f6c0e6]
	[0xb4f6c13c]
	banshee() [0x8062bc8]
	banshee(mono_runtime_invoke+0x3e) [0x8192eee]
	banshee(mono_runtime_exec_main+0xe0) [0x81959e0]
	[0xb4f6c077]
	[0xb4f6bf37]
	[0xb4f6be06]
	[0xb4f6bdb0]
	[0xb4f6bd32]
	[0xb4f6bce8]
	[0xb4f6baea]
	[0xb5739351]
	[0xb573944b]
	banshee() [0x8062bc8]
	banshee(mono_runtime_invoke+0x3e) [0x8192eee]
	banshee(mono_runtime_exec_main+0xe0) [0x81959e0]
	banshee(mono_runtime_run_main+0x11d) [0x8195ced]
	banshee(mono_main+0x1676) [0x80b7706]
	banshee() [0x8059355]
	/lib/i386-linux-gnu/libc.so.6(__libc_start_main+0xe7) [0xb758fe37]
	banshee() [0x8059291]

Debug info from gdb:

Could not attach to process.  If your uid matches the uid of the target
process, check the setting of /proc/sys/kernel/yama/ptrace_scope, or try
again as the root user.  For more details, see /etc/sysctl.d/10-ptrace.conf
ptrace: Die Operation ist nicht erlaubt.

=================================================================
Got a SIGABRT while executing native code. This usually indicates
a fatal error in the mono runtime or one of the native libraries 
used by your application.
=================================================================

Abgebrochen (Speicherabzug geschrieben)