Extruder hitting bed after successful print  

Extruder hitting bed after successful print

I just recently built my MK2. I originally had problems with getting PLA to stick, but followed a procedure that Peter posted where I used Live Z Adjust to get the nozzle within a single sheet of paper of the bed. Then the PLA stuck wonderfully and I printed the PRUSA logo. That turned out just fine. Then I decided to try 3DBenchy. The print got to about 60% and came loose, so I stopped the print by using the menu command to do so. I decided that perhaps I hadn't properly cleaned the bed, so I cleaned it with IPA and tried the Benchy print again. This time, during the calibration, on the point on the right side, half-way between the front and back, the extruder smacked into the bed hard enough to leave a substantial ding. I was alarmed and confused.

I decided that perhaps the problem was that the Z-axis had gotten out of alignment. So I tried doing Z-alignment, where the extruder is raised all the way to the top of the carriage, and one side was a little off. I raised it some more, until both sides were back in alignment, then tried calibration again.

Same problem with the extruder smacking into the bed at the same location.

I tried checking the level with my phone. (There's an app.) The bed isn't perfectly level, but it's damn close and there doesn't seem to be a lot of variation side to side. Just a tiny bit front to back.

I'm sure I've done something foolish somewhere. Please tell me what and how to fix it.

Thanks,
Ray