Prendere lucciole per lanterne

Literally: To mistake fireflies for lanterns

Example: “Se pensi che sia stato io, prendi lucciole per lanterne.” — If you think it was me, you're barking up the wrong tree.

English equivalent: To bark up the wrong tree

Origin: Mistaking fireflies for lanterns is confusing two things that look vaguely alike, a tiny glow for a real light. The Hoepli dictionary defines it as seeing or understanding one thing for another. French has a close cousin, prendre des vessies pour des lanternes (to take bladders for lanterns); the Italian version has no documented first use.