Purposeful fail - effect on Master's admission I am intending to purposefully try to fail a course to stay an extra semester, so I can finalize some research projects which can't be finalized otherwise. My current GPA is 3.82. How badly would that be reflected on my master's admission?
Explore all other options first. Seriously. Talk to professors whom you think to possess some common sense (especially your adviser and the department head if they are among them) explaining clearly what your situation is and what your objectives are and ask for the advice on how to proceed. Resort to the F option only if you see that there is no other way.

As to "masters admission", it depends. If you want to master in math., say, and your F is in "underwater basketweaving" or something else like that, I personally would not even notice, but if you fail algebra, that will be quite a red flag and your explanations that "it was intentional", etc. won't fly with me (also, are you going to include these explanations into your application letter or what?). So, if you follow this line, at the very least be sure to choose a subject people in the field you want to specialize in view with contempt.