The limit is around 8132 or so. It's a limitation on SpecialCells and being able to recognize that number of unique formats.

And I use the SpecialCells all the time. If you come up to the limit, you have too many formats!

Edit: As a side note, Bob's autofilter code should have an additional line at the start..

[vba] Sheets("Sheet1").AutoFilterMode = False[/vba]

.. or whatever the sheet name is. Again, for error proofing.