Special characters in Tcl.  Try strings starting with a -Bad and strings containing [BAD], {, \077, and $Bad. For user input, [BAD] should never be evaluated, \077 should not be turned into a ? and $Bad should not be interpolated. The string -Bad [BAD] \077 { $Bad should be valid user input, should pass through the system unaltered, and if it isn't that's a bug.

Quoting issues. Put some html in plain text fields and make sure the result is properly quoted anywhere it shows up (I use "<b>bold</b>" usually). Look out especially for quoting errors in the context bar and in round trips via an edit form. For fields that disallow html tags you can use & to check that the field is quoted properly. If it is not displayed as & then the quoting for the field is incorrect. (It's not clear whether this should be considered an error but given that data for text fields can come from various sources if it's text it should be properly quoted and we should not rely on input validation to prevent XSS security holes.)

Whitespace input. Check that whitespace is not considered valid input for a field if it does not make sense. For example, the subject of a forum post is used to construct a link and if it is " " it will have a link of <a href="..."> </a> which would not be clickable if whitespace was allowed as a valid input.

Doubleclick.  Make sure that if you submit a form, use the back button, and submit again that the behavior is reasonable (correct behavior depends on what the form is for, but a server error is not reasonable).

Duplicate names.  Make sure that if a duplicate name is entered that there is a reasonable error rather than a server error. Check for insert, move, copy, and rename.