While I didn't find the reason of the error I did find a workaround. I used
a two cells containing 0, and 1 as the source. This allows me to both select
from the list, and type in the numbers.

"Sloth" wrote:

I have a cell with the custom number format of...
"Y";;"N"
I used data validation with "List" in Allow:, "0,1" in Source:, and I
checked "In-cell dropdown". I then put a message in both "Input Message" and
"Error Alert". When I select the value from the list everything works fine,
but when I type in 1 or 0 I get the error alert message. I deleted the
custom number formatting and the data validation works fine both ways. Is
there something I'm missing here, or this a bug?