I've closed a number of PRs where contributor tries to merge one of the maintenance branches into master:
(e.g. 3.6 into master or 3.5 into master)

python/cpython#2901
python/cpython#2784
python/cpython#2678
python/cpython#2427
python/cpython#1968

Would be great if somehow we can disallow this type of PR, or automatically close the PR once created.

From GitHub Web UI, each of these PRs are showing maybe hundreds of commits.
It's inconvenient to scroll all the way to the bottom to find the Close button 😞