This is about the umpteenth thread on this topic. The result so far is:

1. In many cases, the reason is known to be the server script crashing.
2. This in turn can be triggered by various conditions, most of which are unknown.
3. Among the known such issues, though not completely deterministic, is a post by a GUEST containing more than two links.
4. Max has decided that these bugs are not severe enough to justify touching the script; full stop and period.

Trying to detect conditions under which the crash is particularly likely can be a wonderful pastime if you have nothing else to talk about. In practice, we proceed as Mr Red wrote: Always copy your contribution to the clipboard before hitting "Submit".

All threads about this topic should be merged, so that at least new contributions are likely to be highly original - as is Mr Red's server theory. What about Russian spies? North Korean hydrogen?