Localization key error - probably something dumb - please help!

[dt1000]

Hello!
I have recently started to get an error message in the preview window of my UILocalize components:

Number of keys doesn't match the number of values!
Did you modify the dictionaries by hand at some point?
(Image attached.)

Could anybody tell me exactly what this means?
Is it caused by a key that I might have deleted?
If so, how do I find it?
It is most perplexing!

Thanks,
Dan

[ArenMook]

The number of columns must be consistent throughout the localization file. You can't have one row have 3 columns and the next one only 2, for example. Check your CSV.