I did an introduction into the basics of linguistics in secondary school and something my teacher pointed out that a rule of thumb is that common phrases or words are the most likely to break grammatical rules.

He then told me a story about a language that was invented to be perfectly regular, and then there was a generation of native speakers of this artificial language and the first thing that happened was common phrases became irregular.

I believe the language must be Esperanto but I'm struggling to find a reference to this anecdote