I would say it's pretty campaign dependent, too. For me,
Paladins - Meant to be the Big Damn Hero protecting innocents and smiting bad guys. Not tied to the Patrons* or Powers, but gaining their abilities through mysterious means. There is only one oath, and it boils down to Protect Innocents, Nab the bad guy, Protect friends, Protect yourself, in that order. Violating the oath is pretty catastrophic, and frankly meant to be hard to do accidentaly, but atonement is possible.
Clerics - The cleric chose to surrender themselves to the Patrons, and the Patrons accepted and bestowed Their grace upon them. Given the worship of the Patrons is a pantheon, Their representative would have to do something really bad to anger all of them. What would likely happen is certain thematically appropriate spells would be denied or capped until such a grievous sin was atoned for. Even so, they would retain the spells that they had but when cast they would be gone. That said, the few things that would have all the Patrons turn Their back on the cleric would probably mean that the cleric is now getting their power from a demon lord or something. Depending on the infraction, entities may be sent to directly punish the character at the next holy day, and if they survive all is forgiven.
Druids - They are the main mediators between the spiritual and physical worlds. They only thing they could do is permanently alienate a collection of spirits or ancestors. That would prevent them from casting the specific spells that those spirits had granted them. Also make that area of the spirit plane a tad bit more hazardous to travel through.
Warlocks - The power is a trade, tit for tat. If the relationship is broken, then you aren't getting anything more from them. You will need a new patron to develop further levels in the class. Or, maybe the relationship can be repaired if you're clever. Infernals do appreciate cunning, after all. Depending on the patron there may be some entities sent to "repossess" the powers granted, depending on how vengeful they are.
Given that most of these classes possess a high wisdom, I do mention that the character is about to do something that could have significant consequences. I dislike the "gotchas" as well.
* "Patron" is a bit of a loaded word in my campaign. The "Patrons" are basically the gods of the campaign as opposed to "patrons" who are powerful supernatural entities who are willing to bargain with a mortal.