On going Excel problem? Question? Nag?

Ok group, here's one for you all to put your thinking caps, little tin hats, or whatever you do your best thinking in and help me solve this nagging problem with Excel. I like to write Excel spreadsheets, and have since the days of Microsoft Works. As I have progressed over the years, I keep coming up with this one small problem that is driving me totally nuts, because it is random. By that I mean I can have the same exact formula in one cell and not get the little green arrow and in the very next cell I get the little green arrow, telling me there is a problem. Since I know there is no problem, I simply High light all of the cells in that column/row and choose Ignore error, and it goes away for a while. I will tell you up front that most of my formulas are based on one cells being greater than: and usually end with, "") here are a couple of examples: =IF(A4>0,(H4*10%)*A4,"") , =IF(A15>0,D15*A15,"") These happen to be off of the same spread sheet. Most other errors I get, after a cursory glance I can tell what oopsie I made, but this one is driving me almost to the Decaf-coffee pot! Thanks all