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.
"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.