Oh I had one of the scares of my young life not two days ago. I have the head off my 1.5 D so I can paint the block, install the studs and put a new headgasket on it, among other things.
I've been treating that head with the kid gloves, as it's the only one I have and if I warp it I'm screwed until I could find a place I'd trust to straighten it properly.
I had some time to go ahead and clean the face of the head on Saturday, so I merrily spent half an hour rubbing it down with progressively finer sandpaper, starting at 550 and going up to 1500. She shines like a mirror, now.

I pulled out the straight edge, dug out the feeler gauges, picked the .1mm gauge... and watched in abject horror as it smoothly slid right between the head and the straightedge. Aghast, I checked several other spots, in both directions, and by the time I was done I practically felt like crying. No resistance at any point to the motion, just smoothly sliding between the two bits of metal.
I put it away for the day, blowing off another several hours I could have used to advance the cause. With the head that far out of whack, what would be the point?
So I went back this morning, before work, and on a hunch I picked up the still-extended feeler gauge I'd used, looked it over... and damned if I didn't start to laugh fit to wake the neighbors. I had the wrong stupid feeler extended. I'd misread the age-tarnished face of the brass feeler, and used one that was way too thin to be used for such a purpose.
I hunted and cleaned and found the proper feeler, rechecked, and my head is gorgeous now.

I just feel a bit like a traitor, I'm painting a VW block Chrysler blue.