This one is the worst. It kept us completely out of the downstairs bathroom and did some real damage.
The first clue was on the floor: 
The bathroom was fine while I was working at the house, letting contractors in for roof estimates or getting the heating repaired. But Toni quickly noticed an odd fungus growing in the grout between tiles, and it seemed to be growing quickly.
The tiles were cracked and loose and the grout was always wet, which ultimately led to the leak from the toilet supply line.
(yes, that picture shows you something about the paint in the bathroom)
There was a strong steady drip in the back corner, which would then seep down in to the grout and across the floor. It seemed to be the connector on the supply line, but once again was solved only after a second trip to the hardware store for a more complete repair kit.
We didn’t want to touch the floor or use the bathroom, but after multiple cleanings and a few weeks of drying out, it seemed to be fine. I’d only find out the benefits of that leak a few weeks later…