HSTS is a good concept, and all browsers implement it, but there's no way to inspect which domains are flagged with it in your browser. So if you pick one up by mistake when you're testing the feature out in development, it just sticks, and the only way to get rid of it is to clear your entire browser cache or reset it, which is extremely annoying.

Browser makers: Please, please, please implement a way for me to inspect and remove individual HSTS "flags" so testing it doesn't become so painful.