As the title says I have got some wrong outputs with the round() built-in function.
The bug occurs only with certain numbers though. (I've had problems only with the number 2.5, but I assume that there is more than one number that gives wrong output.)
Here are some outputs with the round built-in function:

>>>round(2.5)
2
>>>round(2.51)
3
>>>round(3.5)
4
>>>round(1.5)
2

As you can see the number 2.5 gives 2 instead of 3.