Like Duncan I have had a gravel slope issue in Dragonniere in the Lot in France (probably what the OP termed backfill). I had taken a digging impliment and some additional breathing mix so it was OK. I have also had a couple of dislodged boulder situations with bubbles bringing down debris in Thailand in some new passage with chokes where the bubbles had changed the dynamics around the loose rocks to the point that the choke shifted a bit and dislodged a boulder.. a very hasty retreat!

Are we including collapses in "engineered" passage here? These enlarged/modified sections can bea bit more unstable, and I can think of a few incidents where cavers have had sections come down on them and I assume Duncan can think of quite a few more.