Well, firstly limericks are supposed to be funny, so it's legitimate to use such "mistakes".
The other thing is that such mistakes are normal in children. (I forget the details; any summary of first language acquisition of English in children will tell you). Children first learn the past tenses of the very common verbs (which are mostly irregular); then they learn how to make regular past tenses; and they then often regress to say "Today, I goed to kindergarten", even though they have already learnt "went". Finally, they cope with the fact that there are two ways of making the past tense.
Maybe we ignore it because usually when we hear/see it, it's a transitional form which usually sorts itself out?
Student or Learner