What to do (besides rebooting) when Finder won't navigate to /tmp?
The answer appears to be related to setfile. If one runs getfileinfo /private/tmp:

Tomass-MacBook-Pro:tmp tomasgallucci$ getfileinfo /tmp
directory: "/private/tmp"
attributes: avBstclinmedz
created: 07/30/2016 14:32:36
modified: 01/16/2017 14:56:52
Tomass-MacBook-Pro:tmp tomasgallucci$


In my case, the bundle flag was turned (capital letters mean attributes are turned on, lower case means they're turned off). To resolve the issue I was able to run

setfile -a b /private/tmp


And I could once again navigate to /tmp in Finder.

What set the Bundle bit? I don't know. But life wouldn't be worth living if there weren't at least a few mysteries.

Here's a Super User article that discusses the topic in some detail.