I'm running Geary from git master on Ubuntu 16.10.

If I visit a certain conversation, hit R (for Reply to All) and the begin typing, Geary dumps core with this message:

===
(geary:3188): Gdk-ERROR **: The program 'geary' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadAlloc (insufficient resources for operation)'.
  (Details: serial 42454 error_code 11 request_code 53 (core protocol) minor_code 0)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the GDK_SYNCHRONIZE environment
   variable to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)
Trace/breakpoint trap (core dumped)
===

Mysteriously, this only happens for one certain conversation in my inbox - not others.

I followed the advice in the message above and ran 'GDK_SYNCHRONIZE=1 gdb geary' to get a stack trace for this crash - see below.  Unfortunately there are no function names for the stack frames in WebKit, since Ubuntu apparently doesn't offer debug symbols for that library.

+ Trace 236748