Has anyone experienced regular desync issues with online play?

This is a pretty open-ended question, but we've had a game out for a couple years now and are really struggling with identifying our last couple desyncs. In one case, it even appeared that different computers were rounding positional values differently and causing disparate collisions.

We've hammered out just about everything in terms of ensuring that code is executing the same way on both sides, and have validated that it's not a problem with differing random seeds or player input being incorrectly passed. Is there something else at a high level we should be looking for, and has anyone else experienced the rounding issue (and found a definite solution)?