Originally Posted by Pedaleur

The lock ring shouldn't come off, of course, though even a good mechanic might put it on loosely and forget to tighten it later only to have it come off in the Spanish countryside, miles from a bike shop (er...I have a friend...).

It suggests a lack of attention to detail, so I'd go over all of the important bits, if I were you:

skewers
stem bolts
crank bolts
seat collar
brakes
others?

A proper torque wrench is useful, but without it you can tell if something in ridiculously loose.

Or pay $20 to the good LBS for a tune-up.