Could one of your 30's be a character 30 and the rest numeric 30's?
The SUM() function would still work, but the character 30 would count as a zero.
I don't have Excel on the computer I am using at the moment, but in Microsoft Works, for instance, numeric values are aligned right by default, and text characters are aligned left by default, so one can detect this kind of error at a glance.
Of course, one CAN align all the cells the same way and hence use technology to shoot oneself (or somebody else) in the foot if you want to.
Hope this helps!
