Boeing 707
I believed the legend about how the Boeing 707 got its name because it was such an interesting story, but that did not make it true.
The story was that the wings of the new airliner were swept back 45 degrees and the sine and cosine of that angle are both 1/2 the square root of 2. That is 1/2 * 1.414 or .707 and that is where Boeing got the name B-707.
The problem with that story is the wings are swept back 35 degrees. Boeing said the next civilian aircraft series number was the 700 series and someone in marketing said that 707 was a catchy number.