No more dangerous than a mechanic who misses any other step in a bike build. You'd be amazed at the flaws in even the nicest bikes. Your only line of defense is a good pair of eyes and gobs of experience.

What Trek doesn't want is for Joe Noob to stick a 28 on a "racing" frame and have the wheel lock because a spoke broke and the bridge was too close to the top of the tire. They missed by a couple of millimeters. No biggie. Again, the mech should've noticed...