I run several social media workshops, and I have a section all on its own about hashtags, simple do's and don'ts. My golden rules for using Hashtags; Always research your hashtag, Write it down and Always, always if you have a hashtag which consists of more than one word, Capitalise The First Letter Of Each Word.
Tweets with hashtags get twice as much engagement as those without them, and 55% more Retweets. On top of that, 75% of people on social media use hashtags. - Sprout Social
In saying that though, there are some cases when hashtags have gone bad, whether it's not using capital letters, or a phrase meaning something else entirely in a different country, there's always going to be an amusing Hashtags.
Classic hashtag fails
Working on the film the Hobbit the Swiss PR team put together Hobbit with the abbreviation for Switzerland, CH. It may not have the same meaning in Switzerland, but here in the UK #Hilarious. FYI, this tweet roughly translates to Only one month to go until Lord of Rings.
Research in Motion, or RIM, was used by Blackberry back in 2012 for a recruitment drive, which resulted in an abundance of abusive tweets using the official hashtag #RimJobs. Always, always research your hashtag.