I know this isn't very descriptive, but I don't have many other details. My site was throwing internal server errors like crazy for about an hour today. Forms on the front end were unusable, and doing pretty much anything from the back end was impossible. Deactivating this plugin cleared it up.
Before today, it seemed to work alright. Although when logging in I would occasionally see "2 attempts left", or similar, even on my first attempt.
edit: looking now to figure out if this might have been something else.