There's not really any exciting tales to tell unfortunately, although admittedly it may just be me that finds it less exciting as I can peer behind the curtain.
Some that stand out being...
The time a player found a way to generate infinite money in poker by depositing negative cash, the system just automatically jailed him, they never reported it as a bug and just made multis to try and exploit it - which also got auto jailed.
We quickly patched that embarrassing bug and confirmed there was no further abuse.
KK/Reformer's bug obviously a notable example, they found you could duplicate donator packs by depositing them into your bazaar/display case twice, since they were listed under supply packs and special items, if you listed from both areas simultaneously it duped them. This went on unnoticed for years with much of that being before I was even an admin. It was quite a mess to untangle and correct the cash involved there as much had entered the economy.
One bit of drama that came from that was the fact that they dropped a dirty bomb using illegal cash, I was in the hospital at the time for the birth of my first child so I was understandably not available at work, but once back had to work on reversing and resolving that - some more reading on that all here: https://www.torn.com/forums.php#/p=threads&f=1&t=16082307&b=0&a=0
That all came about after I had only been on the job for a month so that was a fun time.
Whilst disappointing as we expect QA to catch most things it's overall not unusual for exploits on newer systems to be discovered, as that's when they're going to be most vulnerable. Even with all the QA in the world its possible for some things to slip through when releasing a new update, which is why we are glad to have the bug bounty program and are as generous as we are with it, because the alternative in our view is less preferable for both our sake and the players'.