Noticed this bug when working on PYTHON-2395 where MongoClient can create more than minPoolSize background connections.

active_sockets is the count of checked out connections. As part of clearing the connection pool, pool.reset() resets the "active_sockets" count to 0. However, if we reset the count to 0 when a connection is checked out then active_sockets will become negative when the connection is eventually checked back in. For example (pseudocode):

conn = pool.checkout() 
# active_sockets == 1
pool.clear()
# active_sockets == 0
pool.checkin(conn)
# active_sockets == -1

Luckily the only thing active_sockets is used for is maintaining minPoolSize connections. So the only side effect of this bug is that the client might create more than minPoolSize connections.