The most likely reason for this error is you have content defined for a server tags attribute that contains a double quote " character and the tags attributes are also quoted using double quotes.
Many data binding expressions used to set tag attributes contain double quotes within the expression so the tag attribute must be quoted using apostrophes '
These are ok but the following will cause the specified error:
Attrib="<%# Request.Form("somefield") %>"
The reason is that the page parser does not know where the tag attribute ends and assumes the second " character it reaches is the end of the attribute when in fact it's not.
Something you should be aware of is the tag editor in Dreamweaver MX always used double quotes by default and can cause the error quite often, however, if you manually edit the tag to use apostrophes for the offending attribute the tag editor will honour this and not change them back.
The property inspectors for all the cart controls have a button called "Fix Tag..." that when clicked will scan the selected tag and modify it as necessary to cure this error.