Take your pick. There's a remarkable number of things that can cause a server to start returning 500s while the rest of the fleet is fine. Doesn't even have to be that specific server's fault (e.g. database behind it could have reached a connection limit handling the connection pools from all the other webservers, leaving this one in the dust. Fun part from this is it can result in the misbehaving server _moving_ as connections close and re-open)