How do hydraulic flat tappet lifters fail?

It's hard to tell, but it looks like several lobes are going away. The one directly behind the flat lobe in your first picture looks like it is being flattened to me, plus the chunk missing out of the lifter is something I've never seen before. Normally they can fail due to debris entering them, improper break in/oil used, or due to internal failure such as a plunger spring breaking. I am interested to see how the rest of the lifters and camshaft looks when you pull it out.