Here's a problem and solution I just ran into.
For the last couple days I've been trying to use screen like I normally do but was running into a problem. Each time I re-attached, the process was killed after a couple seconds and screen automatically detached. This was really anoying: attach, hit a few keystrokes, then get killed. Rinse and repeat. I also discovered that even without using screen, any process that took a few seconds to execute (like downloading some large images with [color=#00CC00]wget[/color]) were getting killed.
Luckily, I found the answer. After reading the "fcgi problem -- site down" thread, I took a hint and had a glance at [color=#00CC00]top[/color]. Sure enough, a certain cgi had somehow spawned several processes that were frozen there and I must have bumped up against a "max number of processes" limit or something because I [color=#00CC00]killall[/color]ed the bad boys and everything was normal again.