Ok I’ll confess
From memory probably x2 basic errors.
1st one many many years ago was whilst servicing a Feinwerkbau 300su I took my time and found it easy to work on but I made an error with the circlips on reassembly for the sledge system I somehow fitted the circlips on the wrong side and I can’t remember if the rifle wouldn’t cock or wouldn’t fire took me awhile to work out what I’d done and plenty of head scratching doh!!
2nd one many more years ago a simple school boy error fitting a scope and not paying attention yes the turrets were the wrong orientation with the windage at the top and elevation to the left side my excuse was the previous scope I had removed had side illumination hence rushing to remove and refit I’d somehow thought the turret was side illumination luckily I realised before I even zerod which would have been fun