> it won't work if it's not compatible

This is very new behavior in pip. Not so long ago, imagine this:

You `pip install foo` which depends on `bar==1.0`. It installs both of those packages. Now you install `pip install baz` which depends on `bar==2.0`. It installs baz, and updates bar to 2.0. Better hope foo's compatible with the newer version!

I think pip only changed in the last year or two to resolve conflicts, or die noisily explaining why it couldn't be done.