Yes we have. I co-own a Grob 103 and taught in several others. Also owned a G102 many years ago. In my experience, I've seen this happen with cyclic fore/aft loading for example if the ship is tied out in windy conditions with some slop in the tiedowns. Also, if folks pull excessively from the wings to maneuver on the ground. As JJ mentions, it's usually possible to get all of the fittings more-or-less equal by loading fore then aft and turning the appropriate collars. There should be some friction in all 4 collars that prevents the pin/collar (there are two different sub-models) from turning without some reasonable force. If one (or more collars) can be turned with minimal force when the other 3 are engaged there is a problem. Either the through-tube is biased toward one side or one of the collars is damaged. See:

http://www.ltb-lindner.com/g-103-ad-...-conection.pdf
http://www.ltb-lindner.com/service-l...assemblies.pdf