You can make fake achievements, and get a good laugh of your guild mates when they fall for it. My DK just got the achievement Level 10, and all the guildies wrote GZ. I had a good laugh 😉
There used to be a home page you could use to make those fake achievements with, but it doesn’t exist anymore. So you have to install one of two addons. If you use Curse Client, it’s easy. Just do a search for “fake achievements”, and I recommend installing the addon called UnderAchiever.
But don’t overdo it. Use it once or twice, and it’s funny. More than that is boring. And you risk ending up with nobody saying Gratz, when you really hits level 80.
Are you at risk of getting your account banned for this prank? No, not according to what Blizzard themselves have written here:
Don’t use it to get into raids. That will make you stand out as a liar. Use it only for fun, and disclose your prank afterwards.
You can download Curse Client here: http://www.curse.com/client/ – For Mac or PC.