One example, is we had them ssh, download & extract a tarball (the Linux source, but the content doesn't matter). Sometimes, they'd gunzip to stdout. The reaction tells you a lot "lol whoopsie" followed by a quick fix: person knows what they're doing. "uh… what is going on? did I break it?" followed with general cluelessness… maybe not.

That did occasionally break tmux, though.

Part of it was "what are the specs of this thing you're SSH'd into?" and we had one candidate who was adamant the numbers must be wrong: 2 GiB is too little RAM, no machine is that small! Yeah we didn't spin up 128 GiB VM for your interview…