For some reason this is the most common mistake I see from students:
for i in range(0,n):
if i % 3 == 0:
print 'fizz'
elif i % 5 == 0:
print 'buzz'
elif (i % 3 == 0) and (i % 5 == 0):
print 'fizzbuzz'
else
print i
It's not so much arithmetic that's the problem but reasoning about overlapping boolean conditions.