I can recall one instance of bad advice on a particular subject (I don't want to be specific). In retrospect it should have been obvious that the person giving the advice lacked the experience to give it, but it's hard to judge someone's credentials over the internet.
Some of the media recommendations have been bad; of course no recommendation is perfect, but in my limited experience LW's strike rate is worse than e.g. TV Tropes (which may just be a factor of the latter containing a lot more detail and having more contributors).
Of the anime recommendations I've followed up on account of their claimed rationality content, I've yet to find one that repaid the effort.
Several weeks ago I wrote a heavily upvoted post called Don't Be Afraid of Asking Personally Important Questions on LessWrong. I thought it would only be due diligence if I tried to track users on LessWrong who have received advice on this site and it's backfired. In other words, to avoid bias in the record, we might notice what LessWrong as a community is bad at giving advice about. So, I'm seeking feedback. If you have anecdotes or data of how a plan or advice directly from LessWrong backfired, failed, or didn't lead to satisfaction, please share below.