I changed the tires on my bike today since it was just about a perfect day to do such a thing.
I managed to bump my front brake rotor. It was no longer running parallel to the wheel. This was a problem since I would lose braking authority on my front brakes. I went off to Harbor Freight to pick up a cheap dial indicator to determine the runout.
At first, it was around 80 mil. A bit of percussive maintenance that was brought down to 30 mil. The spec is 6 mil (well, 0.15mm, but my indicator reads in thousandths).
The problem is that while it feels like it did before I swapped tires, not I know that something is out of spec. :-|