In a few cases, an old post had a diagram with it, but the diagram is no longer there because of a switch to a new server or something.
I remember one (I'm too lazy to look for it), which involved a jigsaw Sudoku (9x9) where the only digits allowed were 1, 2, and 3. There were to be three of each in each row, column, and jigsaw piece.
The poster wanted some help getting started. I managed to find a way to get the ball rolling, and replied something to the effect of:
Well, I can see one thing. There must be 3's in two of r5c3, r5c7, r5c9. But the latter two cannot both be 3's, so r5c3 must be a 3.
The original poster thanked me and went on to solve the puzzle. But the diagram isn't there anymore, so nobody can figure out what it was all about.