If it's the old PR-RP script you can duplicate money easily. You have to play on two accounts at the same time, have the first one being employed as a mechanic. Get a rental or any other vehicle on the second account, make sure he got all his money on hand. Fix the second account's vehicle with the first account, the money will duplicate when receiving the paycheck.
X offers Y a repair for 15,000$
Y accepts the repair
X won't receive the money yet and must wait for the paycheck to arrive
X receives 30,000 even though he charged Y only 15,000$ for the repair.
Enjoy. I used to make millions on this when PR-RP was still up, make sure you find a way to stash the money so it won't show in your /stats. Figure this out yourself, I'm still using this method on a couple servers with a PR-RP script.
I have a couple more bugs I could tell you, just hit my PM's.