I wouldn't call this the worst, but it has the potential to be up there.
You can find this "error" on Gmail and many other sites right now including stackoverflow in this very field.
Scenario: You have a rich text editor that has most of the features of MS word. Ctrl-B makes it bold, Ctrl-I italics, etc. etc.
But what happens when you hit "tab"? You don't indent. You leave the text field and almost always land on the the submit button. So if you ever hit tab, expecting to indent and then maybe enter to add some vertical space (or you thumb hits the space bar), you just submitted the form!
I know it's rare, and it is technically the expected default behavior. Tab leaves a form input. Enter (or space) on a submit button submits.
However, there must be a better model now that we insist that are text areas behave in every other way like a text editor.
Otherwise, you may accidentally embarrass yourself in writing a quick email. Ask me how I know. ;)
Possible solutions: change the default tab order or confirm a submit on an immediate tab-space or tab-enter.