So this happened years ago at my 2nd real job. I was working at a warehouse at the time and they had a hiring agency (lets call them Scam Co.) that they worked with. Everyone would work 3 months at the hiring agency and then if they were up to snuff they would be hired on fully by the warehouse and join the union.
So Scam Co. had hired me in late August and I was hired on in late November and there was no issue. Until right around tax time they sent out an email from their legal department informing everyone who had been working for Scam Co. to be hired for the warehouse was overpaid $1000 on one of our paychecks in November and were demanding we pay it back or legal action will be taken. I hadn't noticed the big paycheck because my birthday is in November and I had received a few e-transfer gifts and didn't really question it at the time.
I was pissed, don't start out with threats of legal action. If they just told me I probably would have just come to an agreement and paid them back over a few months. Which is what most guys did. However the threat of legal action really didn't sit well with me so I went and double checked my paystub they overpaid on and turns out I was taxed an additional $200-$300 on the paycheck because of said overpayment. It all made sense now.
So I called Scam Co. and booked a meeting with one of the on site managers to discuss it, they happily came in asking me about payment plans and everything. I played along for a bit, but finally as they were about to fill out paperwork for the payment plans, I mentioned to her. “Oh hey, by the way, will I be reimbursed for the extra $200-300 in taxes I got charged for the increased paycheck?” She looked stunned. Stammering for a minute she says she will have to look into it and we will meet the next day.
Not once did I hear back from Scam Co. and my 19 year old ass was just happy I got to walk away $1000 up and didn't really tell anyone else other than my buddies who didnt get overpaid about what happened. I felt bad because a few guys asked me if I had been overpaid too and if I was paying it back to which I just basically mhm'd my way through. Thinking back I definitely should have let my fellow employees know and reported them to the tax man.
Some regrets but hey, I was proud I stood up for myself.