Concur on unused leave / prepaid medical / delay between when pay earned and when pay paid (e.g. you’re paid 10 days after the “pay period” ends, they paid her the prior pay period amount as usual, plus the 10 days).
Also, mistakes do happen. A few years back, I got an email from my company’s HR department basically saying “ooopsie, paid you a referral bonus you weren’t entitled to, if you send us a check right now we won’t have to delay your W2”. I hadn’t even noticed the referral bonus; it was just a day or 2 after my regular paycheck, which was direct-deposited. As it was the end of the year and I wanted to be able to file taxes promptly, I sent 'em the check the next day.
If you’re concerned, put the extra in a savings account for a while in case they do ask for it back.