My colleague asked me how to implement a VLOOKUP formula because he was getting an #N/A error message. I highlighted the cells but I did not receive a tooltip like the one shown in the image below (couldn't post image because reputation is too low). I tried to fix the problem by changing the format of the cells (right click, Format Cells...) being referenced by the formula but I couldn't get it to work. I had him email me the file and then I opened it on my computer and when I highlighted the cells in question the tooltip appeared. Is there some kind of Excel configuration that determines if this tooltip appears? (I might be misusing the word tooltip, I mean that yellow box with the ! in it. Thank you for your help.