Holding middle-click to fly vertically, eventually leads to a crash.
Probably a good idea to add a vertical boundary to prevent crashing (if it can't be resolved or if it's a hard limit).
It should already be limited, but I'll look into it.
Fixed in commit c3718fb. Seems there was no limits to how far you could move the camera; it was just something I intended to do (but never actually did).
I've uploaded V0.0.29.0 which includes this fix - I was going to solely upgrade to the current version of .NET and libraries used by the game, but since this causes crashes and is a pretty easy fix I slipped it in.