I've seen:

1) Incorrect ordering of output "Buzz Fizz"

2) Printing only Fizz or Buzz for 15.

3) Starting at a number different to 1 (say 0)

4) Off by one error at the end (going to 99, instead of 100)

5) Forgetting to print out the number when neither Fizz nor Buzz is output (first output is Fizz for 3)

6) Always printing out the number. (3 Fizz)

and that's the people who can code at all.

Then you get people who can't code, and can't even produce a for loop.

Oh and extra points for arguing about the solution being correct, particularly after prompting that there is a bug "Is it correctly dealing with 15?" is a pretty huge hint that there's a bug.