This script causes expect to coredump:

#!/usr/bin/expect -f
spawn false
wait -i $spawn_id

This comment in exp_close in exp_command.c gives a hint about what's going
wrong:

	    /* at this point esPtr may have been freed so don't touch it
               any longer */

Unfortunately, the esPtr which this comment says shouldn't be touched anymore is
in fact touched in the for loop in exp_close_all in exp_chan.c  This is as far
as I got at debugging this problem before running into the extent of my expect
knowledge.

Note that this simple modification to the script makes the coredump go away,
presumably by causing the channel to be removed from the list checked by
exp_close_all before the script exits:

#!/usr/bin/expect -f
spawn false
set timeout -1
expect eof
wait -i $spawn_id