vlookup yeilds "weird" results...

[markjkubicki]

every so often... and now is one of those times...
a previously valid formula, which yielded totally valid results, is currently returning: gobbly-gook



an example is:

=IFERROR(VLOOKUP(C13,Load_Type,15,FALSE),"")

which for a particular lookup used to return the text "FDB" (which is correct); now I get: 29490.5
or in another cell, instead of the anticipated result "INC", I get 3300.468468

2 notes:
- it's never just a random formula; when it happens, it's ALL of the formulas on the sheet
- the gobbly-gook answer is always consistent -ex: wherever I am looking up the same value, I always get the same result

perplexed...
(but with many thanks in advance),
mark