Citing (or not) a flawed or incomprehensible paper or preprint Is there a good practice of citing (or not) a paper or preprint that you consider flawed (or at best - totally incomprehensible)?

Once I had a problem of that sort. I wrote a paper on a topic, which was not very popular. Even if I was not using other's results directly, I wanted to cite a few papers solving very similar issues. 

Then I had a dilemma if to cite a preprint tackling the same problem, using methods I don't understand (with a feeling that it is incomplete, flawed or just extremely badly written).

Ii that case it is better to:


simply drop it,
cite but make it explicit that you are just mentioning it, not using their results,
or cite making it explicit that you have serious doubts on its content?


EDIT:

By a preprint I understand sth which is archived on arXiv or sth similar.
in what follows, I'm assuming you're correct in your assessment of the flaw. 

The first step is to communicate with the authors. If you mutually resolve the flaw (they accept the mistake, and/or redo the proof), then you either cite the corrected version or mention when citing the paper that it has a flaw and cite your discussion with them as a personal communication.

If the conversation with the authors stall (they're nonresponsive or shift the goalposts), then you can cite the paper and briefly outline the flaw in an appendix (or if you're space constrained, post a brief link to a document available elsewhere). I'm not entirely happy with this last idea, but I can't think of anything better in a space-constrained scenario.

I don't think that incomprehensibility is a good reason for not citing, or citing negatively. You'd have to narrow down the incomprehensibility to something in particular that the authors are not clear on (a definition, a set of cases, or something like that). Again, this should be a last resort if the authors are not responsive to contact - ideally, matters of lack of understanding should be resolved between the authors without it making it into the paper. 

post-caveat: my answer deals mostly with theoretical papers. In more applied works, it might be trickier to determine whether something is "known to be incorrect" or "not known to be correct".