What can happen is the yeast was unable to process all of the fermentables originally. Then either the temperature changed or the few yeast cells that could handle the heavier sugars multiplied enough to finish the job. That's why we use hydrometers.
At least you missed out on a lot of bottle bombs.
Remember one unassailable statistic, as explained by the late, great George Carlin: "Just think of how stupid the average person is, and then realize half of them are even stupider!"
"I would like to die on Mars, just not on impact." Elon Musk