Can what you say on twitter affect your future/career?

[Stryder]

Now a days companies hire people to look through applicants social networking sites (Facebook/twitter etc etc)
If they find out anything that doesn't help the business (stupid posts/indecent pictures/anything that says anything about an employee or ex employer (bad mouthing anybody or the company) you will not be hired or sacked on the spot
Many employers now adays (my current and ex employer) will get you to sign a document stating you won't say anything about the company or employees or posting pics about them or its a breach of contract
So be careful, try to avoid swearing - you may have it set to private but an employer can ask for rights to see it or even have the company follow you - saying no to these may get you into trouble