A user just e-mailed me with a problem. He had to process the results of an existing form which was made up of fields named with invalid characters (one was "first name"). Trying to access the field as #FORM.first name# won't work and will throw an error. So how to access these fields?
The answer is to remember that FORM is not just a prefix, it is also a structure. Structure members can be accessed as #structure.member# and as #structure["member"]#. And that latter format will allow access to badly named form fields (as the name is enclosed in quotes). So, the solution is to use #FORM["first name"]#.
For example, 'first name' might come through as 'first_name'.