This is more of a thought experiment, based on an account from (thankfully) someone else's table. It concerns certain bodily functions, and it may be found to be immature. Regardless, I feel it is worth considering, to head off its use on some unlucky GM in the future, and to contend for the gaming equivalent of the Ig Nobel Prize.
Test case: The party has been captured and restrained through mundane means. The wizard has been gagged, but still has his spells prepared. The player contends that the wizard can still cast spells with verbal components by farting. the player notes that since it had never been established how the character uses verbal components, the player was free to establish that this is how all of the wizard's verbal spells are cast. While the GM noted an immediate means of circumventing this, found the idea rather disturbing, and allowed the party to be teleported to safety, ending the scenario.
It can be argued that the player should not be free to announce this background suddenly. Henceforth the wizard would, presumably, be obligated to only use this alternative verbal component. Certain dietary requirements would need to be observed, but this would likely only be a formality. The spellcasting rules describe verbal components as "being spoken in a loud and clear voice" but 'spoken' and 'clear' and 'voice' are not defined terms by the rules, and the orifice in question was not (ahem) gagged.
I await your peer-review, critique, criticism, and hopefully, expansion on this thesis.
This would require those with ranks in Spellcraft to be able to distinguish a wide variety of flatulence - perhaps too much so.
The wizard has been gagged, but still has his spells prepared. The player contends that the wizard can still cast spells with verbal components by farting. the player notes that since it had never been established how the character uses verbal components, the player was free to establish that this is how all of the wizard's verbal spells are cast.
A fart is not verbal. Verbal involves words. Farts are not words.
"A verbal component is a spoken incantation. To provide a verbal component, you must be able to speak in a strong voice. A silence spell or a gag spoils the incantation (and thus the spell)."
Farting is not speaking. Further, the rules specifically mention that a gag spoils the incantation, and the wizard was gagged.
I also don't think I know anyone that can just fart on command at any moment.