My program, which has eight worksheets, was working just fine...until
today. In column A of worksheet 1 (Roster), there is a list of
employee's numbers, and in column B of that worksheet, there is a list
of employee's names. The program is one with which I schedule
workers, Mon. through Fri., by typing their employee numbers into the
left-hand column of each day of the week.

The other seven worksheets of my program are for each day of the week
(Mon, Tue, etc.). In column A of those worksheets, I type in an
employee number, and in column C I have a LOOKUP formula [for example
=VLOOKUP(A5,(Roster!A:B),2,FALSE] (brackets not included in that
formula) in place, which tells Excel to go to the first worksheet
(Roster), look up the employee number that was just typed in, then
return the name found. That part seems to be working fine.

However, the data validation seems to have gone crazy on me. I can't
schedule the same worker twice in the same day, of course, so I don't
want to be able to have the same employee's number or name twice in
column C (where the names are returned). To prevent me from making
that mistake, I have placed data validation in each cell of that
column, using this in the "Formula" box: =COUNTIF($C$5:$C$27,C5)=1
In the "Style" box, I chose "Stop," and in the "Message" box, I typed
"You have already scheduled him/her!" It all seems to be working fine,
until I get down toward the bottom of the column where I am typing in
their numbers. It will stop me from typing in an employee's number,
giving me the warning "You have already scheduled him/her!," even
though I really haven't. I have gone back up and down the column,
carefully looking to see if I have already used that person's number,
but I really haven't. Then I went back to the Roster worksheet, to see
if there are, mistakenly, two people with the same employee number,
and there aren't. I checked the formulas, including their ranges, and
everything seems to be correct.

Does anybody have an idea of what might be going on, here?