User blog comment:Greatbuddha/Introducing irregularities in a conlang/@comment-3998736-20120911052100

In my bit of experience irregulars aren't too rare and happen most often when you go at something with short term goals for a smaller vocabulary, as I imagine is probably amongst the major sources of irregularities in natlangs next to imported words.

I've had them pop up due to repurposing and generalization of words, following rules on mutation causing a word or series of words to becomes unpronouncable and the making of compound words making duplicates and finally sometimes I end up applying new rules over old vocab.

If you're looking for exceptions yourself, try to break some rules with general phonology; importing loanwords that have trouble fitting in as nicely and then figure out a way around it so it still sounds nice. Try some wierd translation exercises that stretch your ability to use the frame work you have as well. If all that fails, you've probably made just yourself a highly regular language, which does occur in natlangs though to what extent I can't comment on.