How can a read only textbox generate the error "a potentially dangerous request.form value was detected from the client..." ?

Perhaps I am missing something really obvious, but I am puzzled why I receive loads of warnings telling me that a textbox on one of my web pages has been the source of the error  "a potentially dangerous request.form value was detected from the client...". This textbox is simply used to display a value from my SQL Server database and its ReadOnly property is set to True, so how can a malicious user insert potentially dangerous code into it?
David