Camera is easy to clip through linedefs
What it says on the tin. All you have to do is press the camera up against the linedef and press Reset Camera
(by default bound to the R key). Or go into and out of firstperson.
Does not work with FOFs.
Found it just recently while messing around on the S1DM server. As such, it is present in the current 2.2.10 (and likely next
as well).