It may have happened to you.
You are working with Excel for hours and then, maybe after a big paste, maybe after inserting a row, you get the error "Too many different cell formats". Sometimes Excel crashes, sometimes you just can't format the spreadsheet the way you want.
There's a simple tool that clears the unused styles from your file, at XL Geek. I was working on a 5MB file that was crashing rather frequently. I know enough to clean a lot of common things and ended up with 500Kb and a bunch of corrupted styles (see picture above - the 386 grabber is a common one). I used the tool and got to 17Kb. It cleaned more than 6000 unused styles.
It's simple to use and it uses .NET 4.0. If you use Excel frequently and occasionally see the "386 grabber" style, give it a try.