Fix Your TLD Validation
Here's a case of bad input validation which causes problems, several sites I've found use this "one dumb trick" for email validation -- check that the TLD is between 2 and 4 characters long and contains only ASCII alphabet characters.
That validation maybe have been fine quite a few years ago, but with the explosion of new TLDs this naive implementation will become more of a problem.
So, fix it! There are plenty of other ways to do your validation that are actually, you know, valid (or at least better)