One of the many problems with grammar rules — and perhaps one of the largest — is that they have exceptions.

So by the time you’ve memorized the rule, you’ve already spotted an exception out in the wild [the only place that matters, by the way: only real usage matters — your grammar book and even (much as I love them) your dictionary can go straight to heck in a showdown with real usage].

Now you have three problems:

1. the time and energy you’ve already wasted learning a stupidly complex rule, and
2. the time and energy you’re about to waste trying to force the false, mechanical logic of the rule onto the organic logic of the “exception”. To say nothing of
3. the time and energy (your own and other people’s, this time) you’re going to waste misapplying the “rule” and causing confusion and frustration to reign throughout the land.

Language isn’t physics. You can’t plug a verb into a “conjugator function” as if it were some sort of variable and come out with the right answer. This ain’t no F=ma, my guy. But, whatever. You do you, boo.