The most common problem (so I'm told - never risked trying it!) is that some camera's have the self timer on a seperate lever from the M/X switch. In such cases you must NEVER use the self timer in M mode. Self timer only works with X.
Originally Posted by David A. Goldfarb
My Rollei has a single M/X/V switch so its impossible to screw up, but my Yashica has an M/X lever and a timer lever. It's generally recommended that you find a way to jam the M/X lever in the X position to avoid the potential problem (and like you're ever going to use M mode anyway).
I've heard this advice from many sources, and related to many kinds of camera (which is a little odd). Is there some design reason why the problem should apply to a range of shutter designs or is it only some that have this problem, and the advice has been spread further than necessary?