I once reviewed a Pull Request made by a fairly junior developer. They had joined recently, and this was one of the first times they had to touch a bigger part of the code.

Due to a mix of inexperience, new (to them) coding standards and lack of git knowledge, they ended up with a mess of a PR, with a few thousand lines changed, and no way to split it off.
I ended up spending the best part of a day reviewing the whole thing and requesting changes.

Even with the long list of improvements, however, I wasn't sure they would get the magnitude of their fuckup.
So I decided to use a real-world, palpable way to show them what they had done: I went and printed the github diff for that PR. It rendered the glorious amount of 73 pages.

I'll never forget their face, and those of their teammates, when I barged into the room with a thick wad of paper and deposited them on their desk.

At least it worked. I never saw another big, ill-thought pull request from them again.