Hi Jay,
Thanks for the response. It is certainly odd behavior. Both of those titles, while a little long, should work. Even if the title were an issue, it should have failed for your co-worker. I also don't see anything obvious in the x or y columns that would make ArcGIS think it is a mixed data type field (at least from the screen shot provided).
The only other thing I can think of is that maybe you were not including row 1 as headers and they were being treated as values in the column itself? Would match up with the mixed data type issue that Tim mentioned.
This may be one of those things we can't give a definite answer to without a reproducible case.
Jonathan