Mystery Plumbing Mistake

[Dan Corcoran]

My crew helpfully replaced the plumbing lines behind the head and replaced the manual overboard waste pump which was leaking.

When they were done, pumping the toilet causes liquid waste to go our the air vent (the tank is not full)

I cannot imagine what plumbing mistake can cause this, can any of you?

[Vento Solare]

Hand over hand the piping. My guess is they misconnected the vented loop on the overboard discharge. See this thread with some really good diagrams others submitted.