I have several sheets for months that I want to sum up in one sheet. I use
the formula = if(iserror(vlookup..) and so on. The formula is correct except
when the number I want is the first number in the lookup list. (Row 2) The
numbers are NOT in order, not in the monthly sheet or in the summing up, and
the order of customernumber is different in all sheets.

If I try the same in an ordered list, there's not a problem with this.

Ex from the lookuplist:

Column N:
Customer
185449
203920
115109