I fell into one of the holes on the 3rd boss and didn't die, first off. Then, after pressing F6 for some benign reason, it took me to Stage 2's secret room except with a black background, from which I pressed R to take me back to the boss. After starting the boss, I found that I could 1 shot it's first phase, and the rest had no health bar, but could be killed as normal. Also, if I pressed F6 again, it would just reset the boss.
The problem does not persist after reloading it, and when I pressed F6, instead of taking me to the secret room, it took me to before I died on one of my attempts and did not allow me to one shot the first phase nor did it take away the health bar.