iRule with table apparently leaking memory

The SOL is accurate; if you're using the -count flag, that leak doesn't apply. The only other known issue which looks like a leak happens when you have your box configured as an HA pair, but don't have (or can't connect to) the peer. All of the records to be mirrored (including the session table) get backed up, waiting for the peer to acknowledge that they've been mirrored. If that never happens, then they never get freed. If that's not it, then you likely need to open a support case. If that is it, you might want to open a support case any way so that we can track instances of this issue (reference ID 247874).