Lawyer/Banker Question

[Temporary_User]

I have a friend who quit a job 2 months ago.
Payday for him is direct deposit on the 15th and 30th of each month.
On the 30th he received his normal hourly pay.
On the 15th of the next month he received his holiday pay.
On the 30th of the next month he received some money for reasons unknown to him (about the amount of a standard paycheck)
On the 15th of the month after that, he received some more money for reasons unknown to him (>$75)

Is my friend simply the luckiest man alive, or will he have to pay it back? Is there a time period where after he holds onto it for a certain time period it becomes his?
What are the laws regarding this?

[jolinar of malkshor]

Could be unused vacation leave and or any other benifits.

[troutman]

If you get two weeks vacation, you get 4% vacation pay, 6% for three weeks, etc.