This happens from time to time when the server is loaded with a lot of requests. I don't know the intimate details (as I never ran a SMF forum myself) but my experience running a variety of other servers (including a Wiki site, a Database front-end phpAdmin, and my own PHP applications) lead me to conclude that database-back end connections are a limited resource and if many users are requesting a page simultaneously then there may be occasions when requests cannot be replied to in a timely manner.