According to Google, there are 33 examples of "mightened of" in evidence on the Web. (I wonder how many there'll be this time next year?) Although it's pretty stupid-looking, it's forgiveable. After all, how many times have you seen the correct "mightn't've" written down? (Well, only 53 times on the Web, according to Google...) And it looks pretty stupid, too..
Blogpost at Language Log
(Update: It's now 31/8/11 and the current hit total is 114!)