A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Some observations which may have something to do with the problem.
a) I had the same issue with a very simple conditional format - 4 rules setting the background colour of a column of cells depending on the text in the cell. It has always annoyed me that copying and pasting rows in this sheet does not use the original conditional format range =$A:$A) but generates 4 new conditional format rules for the pasted row with range =$A$122, and generates a new range =$A$1:$A$121;$A$123:$A$1048576 on the 4 original rules. After six months of copy/pasting rows there were an awful lot of rules. That may have been the cause of my problem.
b) Before I cleaned up I noticed that the "Stop If True" checkbox was not activated on most of the rules. That could have been the cause of my problem too.
c) The error started occurring after considerable changes to my VBA code, but none on the sheet with conditional formatting. I experience Excel 2013 running into odd sorts of issues like this, and usually get around them by rebooting W8.
Thanks to you guys for this thread - it helped me get round my issue very quickly, as my conditional formatting was easy to clear.