when performing a coin magic utility switch and someone in the audience sees the extra coin hidden in the other hand!
Or you are palming three coins and the fall off!!

What do you say when you get caught?
Got any funny/witty lines ??

"oops" cover face with hands in humorous fasion

"where'd that come from?"

"this is why practice is important"

"what? I didn't see anything." continure acting in denial

throw coins on the table and storm off

do a fall, that always lightens the mood

"you can't prove anything!!" swallow coin, then pull it out of somewhere interesting at your leisure.

"the real magician is coming soon"
"It worked in the magic shop"

Whoops. Sorry about that. Wrong trick.

Craig

From my experience, I would immediately go into a new routine. No need for escape lines; it only shows you've been defeated.

For this example, after performing the shuttle pass with the unfortunate flash, I would hold up the coin and say "See, these coins just seem to materialize out of air.", holding the flashed coin up to their face. Ditch the other coin, use patter and say something like "The coins can also dematerialize." I'd vanish the other coin, and say "They return to my pockets." using sleight of hand to reach into alternating pockets and pull out the coins. Then I would perform 3-fly or anything that follows the patter for the situation.

Do NOT use escape lines. They are ineffective. You want to establish your reputation as a magician, not a silly guy with tricks that sometimes fail.