I pushed `v' over a directory in a `dired' buffer, and XEmacs
disappeared. In "~/.xsession-errors", I found:
xemacs: Fatal I/O Error 14 (Bad address) on display connection ":0.0"
after 32552 requests (32550 known processed) with 3 events remaining.
... After restarting, I tried again, and it's working fine now.