It appears that connections that are opened and then closed during recording are only opened during playback and never closed (until the replay exits). This means that extended recordings for a typical application that opens and closes connections frequently will likely run out of connections. This also creates a load consisting of ping commands that wasn't present in the recording.

Example: app has 10 threads that each open a connection, insert a document, then close the connection. During recording (A-B) there were never more than 10 connections open at a time, but during playback (C-D, E-F) connections appear to accumulate indefinitely, up to 20 k connections in this run. (Note that the replay is also creating connections at twice the rate they were created on recording; I think this is TOOLS-1499).

It appears that this is because connection open/close is not recorded as an event in the playback file.