There are undoubtedly tons of people who think they know what their tattoo says and are dead wrong.
I have a lot of friends in the tattoo biz, and the exception to the rule is for someone to go to the trouble of finding the kanji character they want in an actual dictionary.
Oftentimes, people rely on much more questionable sources, like bringing in any number of items they bought that already have a kanji character on them that is supposed to mean a particular thing.
Case in point, one of my tattoo artist friends was given a coffee mug years ago that supposedly had his name in Kanji on it. This was before the current explosion of popularity of the stuff, and he was a young tattoo artist, so he decided to tattoo the character on his leg. It turned out several of his relatives had received mugs bearing identical characters, all of which were supposedly representative of their individual names. To this day, he has no idea what the character tattooed on his leg means.