Doomsday Engine

First I thought that my savegames were corrupted as I would always fall off
(or so I think, based on what it looks like) the map immediately after
loading a saved game. This happened multiple times, but not always. Now I
can easily reproduce a situation where I don't fall of the map, but a lot
of objects do (or maybe they just disappear, if it's another unrelated
bug), making it impossible to continue.

Start a new game in Shores of Hell. Save immediately. Load. Start
playing. The first group of enemies is there, but as soon as you proceed
to the next room, you should notice that there are very few opponents. No
shotgun on the red thing in the room after the first portal. Oh, and no
keys, except the red one in the secret area. Oh, and after going through
the first portal, all portals stop functioning. Can't go anywhere, can't
get keys. The only way to finish each episode is by paying in one sitting
without dying & reloading at any point.

64bit Gentoo, deng-1.9.0_beta6.7. The problem was present in beta 6.2
also; other versions I have not tried.