I think it's a combination of impatience and the server load.
I have had the 500 error cause me to post multiple copies of a message, and get the 'flood control' message a half dozen times or so.
I wonder about some of them though that have time stamps 5 minutes or more apart. I've even seen one that was nearly 24 hours between duplicate messages.
Most of the time I think it's more a matter of thinking it didn't submit on the first click because it takes sometimes thirty seconds or so for a message to post. Clicking submit again thinking it didn't submit the first click seems to me to be a more likely cause.
[This message has been edited by Weird Harold (edited 05-07-2000).]