In TF2 (in the 2014 Halloween map, where I have to deliver the tickets, for what it's worth), I keep getting destroyed by a mini-sentry, but when I die, instead of zooming in on the sentry that killed me, the camera zooms in on what appears to be an invisible sentry. I've tried launching rockets at this thing, but it was to no avail. Is there any way to destroy these invisible sentries?
There was a bug that was allowing people to build sentries outside the bounds of the map.
Supposedly this was fixed in today's update:
Fixed players using the tiny-melee-only curse to get outside of the playable area of the map