ASP.net RequiredFieldValidator not preventing postback

Question

I have a question about what could stop a RequiredFieldValidator from preventing a postback.

I began working on an older, but simple aspx form and my predecessor use

Answer 1

Whew. Okay, I found the problem, basically by creating a brand new project and comparing its web.config line-by-line with my old project. Turns out the culprit is this:

If I remove the line, my validation works the way I expected it to. Googling that uncovered a bunch of blog posts about how VisualStudio adds that line to the web.config when upgrading web apps from .net 1.1 to .net 3.5.

The blog posts were mainly complaining about how that field interferes with .net's AJAX stuff, but I'm guessing it messes with the JavaScript emitted for the RequiredFieldValidator in a similar fashion.