catch someone red-handed

「悪いことをしているところを捕まえる」。手が赤いのは、なんとなくヤバそうなイメージではあります。

id Sports Illustrated Try to Game Dig? on Mashable

Poor Ben Kaplan. The Sports Illustrated Social Communications staffer has just been caught red-handed trying to place his hands in Digg’s big cookie jar.

"Hands in someone's cookie jar" means meddling with someone's business.