A combo's rowsourcetype=1, columncount=2, and the rowsource is built as a long string, like this:
.rowsource="first string,1,second string,2,third string,3, ... tenth string,10"
The controlsource is actually a character (but may be a numeric - and I didn't try to see what happens then). Now comes the weird part: when the length of rowsource string exceeds 255, the last entry before the boundary matches a blank value.
I think it's because it's missing its pair, the value for the 2nd column, which is by default "". Since it's the last one, it's matched if nothing else matches.
This is not a problem to fix - IMO, the hard part was to catch it and understand how it happens. I do hope to go down in history as the guy who found the LAST combo's bug :) (IOW - it'd be so nice if there weren't any more).