Naturally, there are limits to the amount of queries that can be made, so the goal isn't to try and get them "unlimited". Common causes of query overloads are either 1) an infinite loop in your code, or 2) a "phantom" file or files which can create endless duplicates to be made (normally for backup). It is also possible that a site gets shut down by SQL injection, a form of attack, though it isn't...likely.