I've had/seen this happen with certain combinations of 1911 and holsters, including a duty holster or two. I don't like big safeties anyhow so my 1911's invariably have the GI or Colt commercial style safety. I won't tolerate a mushy safety or one that takes undue effort to switch either direction.
These days I IWB with a Safariland 27 and I've never had the safety come off, even after long days.
“Nine-tenths of tactics are certain, and taught in books: but the irrational tenth is like the kingfisher flashing across the pool, and that is the test of generals. It can only be ensured by instinct, sharpened by thought practicing the stroke so often that at the crisis it is as natural as a reflex.” ~ T. E. Lawrence