I put too big a return pump on a predrilled 125 once. I figured the split in the return was going to cause considerably more head loss than it did. Wet carpet.
so I drained the tank, and moved it across the room, got a steam cleaner and had at the carpet, then ran a dehumidifier in that room for a few days, then moved the tank back, set it back up, filled it back up, and ran it with the new pump. works awesome. So I turn the pumps off so I can work on a baffle in the sump and don't realize that the person who owned the tank before me glued the siphon breaks closed in the return. I remember thinking, wow, that water level looks really low. Then I heard water cascading from somewhere. (the sump) I had no idea wtf was going on, or how to fix it. I was in full panic, invisioning 125 gallons of water in my downstairs neighbors living room.
Something in my brain said "turn the pumps back on, it worked when the pumps were on" so I did. Then I figured it out. Wet carpet.
drain, clean, repeat.