A fantasy heartbreaker is what you get when an enthusiastic amateur who has only ever played D&D tries to design an RPG. Ask your GM, they probably have one.
Once upon a time, those might get self-published and spend the next few years drifting towards the back of the game store's shelf, then the bargain bin. These days you'll find them on home-made ungoogleable websites, as unrated pdfs in online stores, and on Kickstarter.
They unquestioningly copy D&D in most things, but add in a handful of original (or independently reinvented) mechanics and/or a very peculiar setting. It's seeing those nuggets of creativity buried in unsalvageable D&D-flavored shit that causes the heartbreak.
Term coined in 2002 by notorious Forgefag Ron Edwards.
- Standard fantasy setting, which they used to feature.