I've always loved the idea of playing a warlock that presents themselves to the party as a cleric (and then seeing how long it takes people to catch on, both in and out of character)
Nothing like driving the "cleric" deeper into debt to their patron for a little more healing and then getting the whole party committed to some insane quest at the demand of the patron.
Yeah, I know. It would probably take some help from the DM to homebrew a patron that has cleric-flavored cantrips, and even then it would likely only work for a few sessions at low levels, but I still like the idea