Do not send, download, or exhibit pornography, sexually explicit jokes, or inappropriate screensavers

You might think this is harmless and just for jokes between yourself and your coworkers but if someone else sees the joke or screensaver and gets offended, you are definitely going to be in deep waters and will probably get fired. Anything that poisons the work environment is a pretty serious offense. The work environment should be professional.

http://money.usnews.com/money/careers/articles/2008/03/11/5-ways-your-computer-use-can-get-you-fired