Perhaps that's the subtle humour behind this linguistic change? "I broke your mug on accident", at the fifth word, the listener/reader thinks the other person

Perhaps that's the subtle humour behind this linguistic change? "I broke your mug on accident", at the fifth word, the listener/reader thinks the other person is about to say "on purpose", but suddenly they say "on accident". Hence it's funny, reduces the tension about them breaking your mug.