Some users tend to pass wrong options to a program on the command line, to define sections in configuration files that nobody cares about (e.g. 'vehicle/front-tire-groop') and tiny input errors can prevent a program from functioning. In order to allow the program to tell the jittery user that he did something that nobody understands, GetPot provides UFO detection !
Indeed, these unidentified command line arguments that fly around without ever being processed, these variables in configuration files that do not serve any purpose, command line switches that do not switch anything - these things are easily detected and the programmer can decide what he wants to do about it.
NOTE: UFO detection is prefix dependent ! In case you were using prefixes, you must reset the prefix to the section you are investigating5.