I'm officially declaring that I am the one who coined the phrase "pulling a singing Frog".
Definition: to be said of a phenomenon that occurs only when a very limited number of people (normally only one) are watching, that stops or does not occur when that limited number tries to show the phenomenon to others.
Examples: my heart beats erratically when I am alone or with my girlfriend but behaves perfectly when plugged into an ECG, or in presence of any health professional ; a computer problem that happens all the time, except when you want to show it to a technician ; a faucet that leaks up until 5 minutes before a plumber arrives to fix it, and starts leaking again upon frustrated departure of said plumber.
Origin: the singing frog, as you might be aware, does originate from the 1955 Looney Tunes episode, One Froggy Evening. The Michigan J. Frog would sing ragtime tunes to the man who found him, but would would croak merrily when the poor guy would attempt to show his talents to anyone else.
First time used (by me): Approximately 2 or 3 months ago, when refering to my heart problem and the way it behaved with doctors (see first example).