[jwr]

My computer has at least three different Python versions installed. I'd say that's a pretty big backwards compatibility problem.

There is also Perl 6, and I've recently read about Go 2. Swift changes every couple of months in incompatible ways.

[xapata]

I also have several Pythons installed. For me that's because of incompatibility between community packages I've installed, rather than differences in Python versions. Unfortunately, the various smaller communities can't always stay in sync. Right now there's a conflict between XGBoost and NumPy using MKL on MacOS. That's not a Python problem, exactly, as the issue is conflicting use of OpenMP, written in C, C++, and Fortran.