Here is what I am thinking of:
- Apache restarted - Some hosting companies will do that if it reaches a CPU usage limit.
- Some hosting companies will limit your processes to x number of processes at the same time (ex: 1and1). So once three people access your site, everyone else will get error 500.
Some hosting also have ignore_user_abort/set_time_limit in their disabled_functions list for that same reason.
Update: Changed Many to Some. Guess the hosts I use are pretty different than what people normally use.