One that is well known is a sort of "spam filter" certain words at certain times can cause posts not to take.

Points 2 and 3 by Max in the 503 error thread may be relavant.

"2. Spam is increasing. We get hammered every day with thousands upon thousands of attacks from people and worms and spiders trying to use our site for nothing we want. We usually have more manpower to deal with it, but:

3. We're down a very key member of our staff due to an ongoing family emergency. Our planned upgrade for the 10th anniversary includes a great deal of behind the scenes defense systems, but is delayed due to this issue that I will talk more about pretty soon."