Rob Brooks-Bilson blogged about an issue he's seen when using qForms with Mach-II. Seems that Mach-II stores all it's generated content in a variable called "Request.content." However, when running in Debug mode, the Request.content code gets outputted a second time, thus causing errors in your template because of the duplicate output.
I'll try playing around w/the code to see if I can stop qForms from throwing a fit, but this is really more of a problem with the debug templates. I normally edit CF's debug templates to make sure that output is wrap in the htmlCodeFortmat() function—that way I see actual value of the string—and not what it's rendering. One of the comments on Rob's blog also mentions this point.