Have you experienced any in your career?
I've seen 2 actual and a few clumsy attempts.
The first was when I first started in payroll and an admin for our sales department would submit a list of route/delivery payments to payroll weekly, with no one signing off on it. She would cash the checks for some of these employees and they would split the money. A few people got fired for that one as it went back for several years.
The second one was through AP Someone submitted several fake checks that they printed at home. The largest was for around 50K. This was before heat changing signature line, watermarks and positive pay.