A canary is an EXA that is intended to crash or risk crashing in order to find information about a system by doing so.
The term comes from the use of a caged canary by coal miners. If carbon monoxide built-up in the mine became dangerous, the canary would die before any miner did; so the canary's death would warn the miners to leave.
The most common use of a canary is when you do not know the number of a file to read or link that needs to be followed. Grabbing a file that does not exist or following an invalid link will crash the EXA, but if the EXA is disposable and created using REPL this isn't a problem.