Larry Tesler championed consistency between applications, and made many contributions to what eventually became the Macintosh User Interface. He was also the leading advocate and implementor at Apple of user testing: actually trying out our software out on real users and seeing what happened. Starting in the summer of 1981, Larry organized a series of user tests of the nascent Lisa software, recruiting friends and family to try out the software for the first time, while being observed by the Apple designers who recorded their reactions.
When the software required confirmation from the user, it displayed a small window called a "dialog box", that contained a question, and presented two buttons, for positive or negative confirmation. The buttons were labeled "Do It" and "Cancel". The designers observed that a few users seemed to stumble at the point that the dialog was displayed, clicking "Cancel" when they should have clicked "Do It", but it wasn't clear what they were having trouble with.
Finally, the team noticed one user that was particularly flummoxed by the dialog box, who even seemed to be getting a bit angry. The moderator interrupted the test and asked him what the problem was. He replied, "I'm not a dolt, why is the software calling me a dolt?"
It turns out he wasn't noticing the space between the 'o' and the 'I' in 'Do It'; in the sans-serif system font we were using, a capital 'I' looked very much like a lower case 'l', so he was reading 'Do It' as 'Dolt' and was therefore kind of offended.
After a bit of consideration, we switched the positive confirmation button label to 'OK' (which was initially avoided, because we thought it was too colloquial), and from that point on people seemed to have fewer problems.
From folklore.org - Anecdotes about the development of Apple's original Macintosh computer, and the people who created it.