Comment 8 for bug 753269

The X11 errors should be reported to the console (or .xsession_errors) but we disabled that.
There's a call to XSetErrorHandler in Unity2dPlugin::initializeEngine that basically sets up an X11 error handler that suppress all errors. We should really remove that at some point and install error handlers that suppress errors only around these parts of code that we know may generate errors. However the fact X11 is async makes it a bit more complicated than that.
Anyway, if you remove that line you will see the errors on console regarding this specific issue.

Regarding the other X11 related bugs, I am investigating a few of them and looks like the cause is quite different from this bug, so I wouldn't suspect a common root cause.