I've noticed an occasional problem with 21.5, where some other
application has the input focus, I click inside an XEmacs frame to
give it focus, and my next keypress produces a "Selection Aborted"
error.

I've seen this ever since I started using 21.5 (many months ago), but
intermittently.  This week I noticed that it only happens with Compiz,
not with Metacity.  This is on Ubuntu Linux, both 9.04 and 9.10.  It
may have also happened on openSUSE 10.<something>, but I don't
remember for sure.

I also got a sample stack trace:

Debugger entered--Lisp error: (error "Selection aborted")
  signal(error ("Selection aborted"))
  cerror("Selection aborted")
  apply(cerror "Selection aborted" nil)
  error("Selection aborted")
  mouse-track(#<keypress-event control-A 0x44da>)
  call-interactively(mouse-track)
  (dispatch-event "[internal]")