One common complaint about "bad" email validators is that they don't allow plus signs in email addresses. Sophisticated users use plus signs to have "different" email addresses delivered to the same inbox: email@example.com and firstname.lastname@example.org both go to email@example.com.
It occurred to me though, that the prohibition against plus signs might not be an oversight, but an intentional attempt to prevent one user being able to register with multiple email addresses on a single site. In other words, the very characteristic of plus signs that make them appealing to geeks also make them a useful tool for spammers. As CAPTCHAs show, spammers must be thwarted even if it inconveniences some honest users.