1. When you have every intention of meeting "that" guy for a drink...but don't want him to asume it.
"Ok, I may show up
accidentally on purpose"
2. Deliberately, but feigning (not by) accident.
Origin
The first printed record of this phrase is from The history of the famous preacher Friar Gerund de Campazas, by José Francisco de Isla, 1772:
Tell us what is Modesty of Voice, for you happened
accidentally on purpose to drop this word, and
I don't rightly know what it signifies.