Following are the first of numerous common "fails" by Twitter users -- labeled as such primarily because they impeded growth or simply do not ultimately accomplish anything useful. Note that I am posting these for the benefit or individuals, businesses, and other entities that have a message they want to get out to an audience. If you don't care whether people follow you on Twitter, whether you can reach them about what you are doing, etc., then the following information does not apply to you, because it is kind of irrelevant to the world at large whether you are on Twitter or not.
* Talking about things that are "coming soon"! It is hard enough to get people to genuinely pay attention to what you are doing and to get out the word about your existing products, projects, etc. Promises about things that do not yet exist, especially if they appear in your bio, look justifiably weak and just make the people who include them look silly — and this is all the truer if the promises are about something like a crowdfunding project! No one will ever say, "Oh, wow, these guys are going to have a Kickstarter 'soon'! That really makes me want to keep coming back and seeing this message again and again."
* Misspellings in your bio. This is not particularly important in personal accounts but can be the kiss of death in professional or business ones. No one is going to buy anything from you if you can't spell, especially as this will raise the suspicion that you are running a scam and don't really have a real business at all.
* Failing to promote your own interests. It is amazing how many people indicate some economic motive for being on Twitter but will not actually take the most rudimentary steps to serve their own interests. If someone claims to be a "freelance fantasy artist" but does not follow back a fantasy game publisher, then you can be pretty sure that they are really much more likely to be a Starbucks barista. And a Twitter account that has something as blatant as a Kickstarter project associated with it that does not immediately follow back every legitimate follower is simply poorly run -- and if they can't even run a Twitter account properly, how can they be trusted to manage the project for which they want your money?