Including changes that are unrelated to the change being PR'd.
- You've written some additional documentation... and also formatted many of the files in the repo.
- You've included a refactor along with a functional change.
- You've implemented feature A and also documented feature B... but the docs for feature B have problems that prevent the merge of the PR
- You've implemented feature A and feature B, resulting in it being hard to differentiate which code is supposed to have what effect.
Sometimes maintainers will have to carry your PR (to carry is to take your PR and make additional changes before merging). Merge commits make it harder to work with your commit history.
Large, unannounced changes are unlikely to be merged unless architectural discussions have happened beforehand.