2003/2007/2010 compatibility puzzle!

Hi,
New member Mike Basingstoke here.
Got a problem since I upgraded (yesterday) to Office 2010 whereby a customer who uses Excel2003 who could share my files created in Excel 2007 now gets #NAME? errors. The functions that seem to be triggering this are nothing esoteric - just tame stuff like IFERROR and INDEX and MATCH. I've researched this in help and on Google extensively but I cannot find a fix. Can anyone help? Much obliged. Mike

Thanks AlKey for a very fast response. I had IFERROR in the spreadsheet for ages and it seemed to cause no problems. I only recenly added a price lookup table that is accessed by INDEX & MATCH (my first time using these after reading a lot of advice that says they work better than LOOKUP) and I've got 609 occurences of significant loss of functionality ! I may just have to copy the cell contents and paste as values. I only upgraded 'cos I'm going on a VBA course in 2 weeks and wanted to be on a newer version that 2007 lol. Have a good day over there in Lakeland - got some family in your town. Mike

Just for closure, I substituted =IF(ISNA(L6),0,L6) in place of =IFERROR... and the whole spreadsheet came good. The problem was not as bad as I first thought because so many cells depended on information rippling through from just this one function. It's still a mystery to me why this has worked fine with my 2007 fies being read by a 2003 installation but it should fall over when I went to 2010. Oh well! Thanks again AlKey.