[pjmlp]

One example sorted out with Python 3 would be old style classes and new style ones.

Other would be tracking down if attributes get dynamically changed via __dict__, __slots__, and how __slots__ interact with each other if multiple definitions happen to exist.

The way numeric division and remainder changed between Python 2 and 3.

[dagmx]

I think you’re talking about a different issue here.

The person you replied to previously was talking about how certain new C++ features don’t work together. For example move with a lambda can cause UB.

You’re talking about changes to the language or if you have a conflict of definition.

[pjmlp]

All of that are changes/features regardless of which form they take.

[dagmx]

No.

They’re talking about feature incompatibility within a single version.

You’re talking about feature differences between different versions.

Completely different things.