frequent msyql crashes after upgrade from 5.1. to 5.6

[msadm]

We recently upgraded from mysql 5.1 to 5.6.19-67.0-56. Since the upgrade we've been seeing crashes somewhat frequently.

Below is information from the last two crashes

15:01:06 UTC - mysqld got signal 11 ;
This could be because you hit a bug. It is also possible that this binary
or one of the libraries it was linked against is corrupt, improperly built,
or misconfigured. This error can also be caused by malfunctioning hardware.
We will try our best to scrape up some info that will hopefully help
diagnose the problem, but since we have already crashed,
something is definitely wrong and this may fail.
Please help us make Percona XtraDB Cluster better by reporting any
bugs at https://bugs.launchpad.net/percona-xtradb-cluster

key_buffer_size=1073741824
read_buffer_size=8388608
max_used_connections=84
max_threads=1002
thread_count=9
connection_count=9
It is possible that mysqld could use up to
key_buffer_size + (read_buffer_size + sort_buffer_size)*max_threads = 25688954 K bytes of memory
Hope that's ok; if not, decrease some variables in the equation.

Thread pointer: 0x5aba370
Attempting backtrace. You can use the following information to find out
where mysqld died. If you see no messages after this, something went
terribly wrong...
stack_bottom = 7fac415e5d38 thread_stack 0x40000
/usr/sbin/mysqld(my_print_stacktrace+0x35)[0x8f36b5]
/usr/sbin/mysqld(handle_fatal_signal+0x4c4)[0x6635c4]
/lib64/libpthread.so.0[0x3bc200f710]
/usr/sbin/mysqld(_Z11alloc_queryP3THDPKcj+0x59)[0x6e3d39]
/usr/sbin/mysqld(_Z16dispatch_command19enum_server_commandP3THDPcj+0x120c)[0x6ef2bc]
/usr/sbin/mysqld(_Z10do_commandP3THD+0x1e3)[0x6f0f73]
/usr/sbin/mysqld(_Z24do_handle_one_connectionP3THD+0x17f)[0x6b9bdf]
/usr/sbin/mysqld(handle_one_connection+0x47)[0x6b9db7]
/usr/sbin/mysqld(pfs_spawn_thread+0x12a)[0xae9b4a]
/lib64/libpthread.so.0[0x3bc20079d1]
/lib64/libc.so.6(clone+0x6d)[0x3bc1ce8b5d]

Trying to get some variables.
Some pointers may be invalid and cause the dump to abort.
Query (0): is an invalid pointer
Connection ID (thread ID): 3449973
Status: NOT_KILLED

15:33:14 UTC - mysqld got signal 11 ;

key_buffer_size=1073741824
read_buffer_size=8388608
max_used_connections=23
max_threads=1002
thread_count=9
connection_count=9
It is possible that mysqld could use up to
key_buffer_size + (read_buffer_size + sort_buffer_size)*max_threads = 25688954 K bytes of memory
Hope that's ok; if not, decrease some variables in the equation.

Thread pointer: 0x508c780
Attempting backtrace. You can use the following information to find out
where mysqld died. If you see no messages after this, something went
terribly wrong...
stack_bottom = 7ff549ab8d38 thread_stack 0x40000
/usr/sbin/mysqld(my_print_stacktrace+0x35)[0x8f36b5]
/usr/sbin/mysqld(handle_fatal_signal+0x4c4)[0x6635c4]
/lib64/libpthread.so.0[0x3bc200f710]
/usr/sbin/mysqld(_Z11alloc_queryP3THDPKcj+0x59)[0x6e3d39]
/usr/sbin/mysqld(_Z16dispatch_command19enum_server_commandP3THDPcj+0x120c)[0x6ef2bc]
/usr/sbin/mysqld(_Z10do_commandP3THD+0x1e3)[0x6f0f73]
/usr/sbin/mysqld(_Z24do_handle_one_connectionP3THD+0x17f)[0x6b9bdf]
/usr/sbin/mysqld(handle_one_connection+0x47)[0x6b9db7]
/usr/sbin/mysqld(pfs_spawn_thread+0x12a)[0xae9b4a]
/lib64/libpthread.so.0[0x3bc20079d1]
/lib64/libc.so.6(clone+0x6d)[0x3bc1ce8b5d]

Trying to get some variables.
Some pointers may be invalid and cause the dump to abort.
Query (0): is an invalid pointer
Connection ID (thread ID): 11855
Status: NOT_KILLED

Any ideas as to what may be going on here?

[msadm (msadm)]

An Update:
We were seeing mysql crash every 2-20 minutes. We were using the Percona XtraDB Cluster version 5.6.19-67.0-56 as we were planning on setting up a Galera cluster in the future. We uninstalled Percona XtraDB Cluster and installed Percona Server 5.6.17-66.0 and have not seen mysql crash since. I can't say whether it was the Galera bits or the slightly newer version of mysql that was causing the problem.