Sirius’ death
Sirius’ death, although arguably the most heart-wrenching in the series, is also the most unnecessary. Gee, if only there had been a way for Harry to prevent Voldemort from implanting that false vision. Oh, wait, there was. If only Harry had had some sort of magical object to see that Sirius wasn’t actually in danger. Oh, wait, he did. If only Harry could have put the brakes on jumping to conclusions, he’d still have his beloved godfather around. But no.