The creators of an iPhone calculator application have installed a profanity filter in fear of falling foul of Apple's stringent App Store policies. The company behind PCalc won't let you use your calculator to spell out rude words.
If you're confused, think back to your school days - typing in 5318008 and then turning the calculator upside down and having a bit of a giggle, then trying to explain what's going on to an angry teacher who knows exactly what's going on.
Apple is very strict about 17+ content on its app store. App developers are often fearful of their apps getting rejected for even a hint of "inappropriate" content. The profanity filter is active in both the paid-for PCalc app and the free PCalc Lite application.