The Irregular Verbs don't follow the rules as we have learned them.
# consonant changes (I bend, I bent, I have bent)
# consonant added (I mean, I meant, I meant)
consonant added + change in vowel sound
(I keep, I kept, I have kept)
# vowel changes (I fly, I flew, I have flown)
# change in word (I go, I went, I have gone)
# don't change at all (I hit, I hit, I have hit)
# ought/aught (I buy, I bought, I have bought)
# there are over 200 of them!