This is a unique bug,
I'm brewing two identical batches of beer, and have two Tuns next to one another. I filled one Tun with water to heat the mash, took the hose off and attached it to the other to fill it with water and do the same. But this wouldn't fill. Nothing went it. So I look back over at the original tun, and despite NO HOSE BEING CONNECTED TO IT it was filling with water.