On Saturday I made a Belgian Tripel. It's fermenting with WY3787. I had a blowoff tube attached right after pitching. But once it stopped bubbling furiously (after 2 days) I assumed (wrongly) that the krausen had fallen. At that point none at all had come through the blowoff tube into the bucket. So I replaced the blowoff tube with an airlock. Bad move.
The krausen had actually risen. But I didn't check under the lid when I replaced the lock so I didn't realize it.
When I was at work today a bunch of foam erupted from my airlock. Luckily it did not clog and explode, but rather it got all over the top of the lid to my bucket, and some on the floor.
Anyway, what I learned here is NEVER replace your blowoff tube prematurely (heh that sounds funny). But seriously, leave it on until you are absolutely sure the high krausen is over.