We have covered the topic of someone else's Google web history showing up as yours. Including one parent catching her son searching for pornography. But what if you find your web history showing searches you did not do personally and you are convinced no one has access to your computer.
There, Googler, Jimmy Deheeger said that in some cases, someone may be watching you with keylogging software. The keylogging software can watch you as you type in your passwords and then these evil people might login as you and conduct searches that make you unhappy. Why would a person spend the time to use keylogging software, simply to run pornographic queries on someone else's account makes little sense to me, but Google suggests it as a possibility. I mean, the person can easily sign out and no web history would be tracked. I don't get it.
In any event, Google offers this up as a possible reason why web history that is not yours would show up in your Google account.
Forum discussion at Google Web Search Help.