I prefer a little explanation here.
"I've just had to track down bugs due to globals defined in this context when they don't need to be. If you use this approach it will save everyone a lot of time in the future."
Not confrontational, it's educational, and you're saying why it's wrong. Just saying "it's wrong" doesn't help if the original author didn't realize it was wrong at the time. If they did realize it was wrong and did it anyways, giving the full description just reminds them why they should go and do it the long way.