Rats, Day 2
This job has the most variables of any in Payday 2, but you can be fully prepared by just anticipating a gunfight. When you arrive to trade the meth for money, you can either get what you came for, receive extra cash, or find yourself in a shootout.
The best case scenario is to trade your meth, receive payment, and leave - any additional meth results in additional cash - but this doesn't always happen. If things go south, you'll have to kill off the gangsters get information from a safe within the building. Fortunately, you can hold off any cops without too much difficulty from most rooms inside.
You can also steal back the meth you brought if things go poorly. This will net you even more cash at the end of the job. Finally, a stealthy approach is actually possible if you ignore the gangsters and break into the safe without detection. If you choose this final, difficult, stealthy path, just be sure to stay look and check around the many corners inside the building.