I was trying to remove items from my cart by setting the quantity to zero instead of using the "remove" checkbox, so customers could remove items either way.
In shop$db.asp, in the ValidateQuantity routine, there is this code around line 1475:
If getconfig("xallowdecimalquantity")<>"Yes" then
If quantity<1.0 then
For those of us with xallowdecimalquantity set to "No", this sets the zero in the cart quantity back to 1. I changed "quantity=1" to "quantity=0" and got the result I wanted.