Since when were you under the impression of x, is a meme coming from Tite Kubo's manga Bleach, generally to mess with any given situation with a bit of troll logic, asking when the subject believed he was correct in whatever the macro is posted as, and occasionally then showing said subject was horribly incorrect.

Origin

The line is delivered by the main villain of Bleach, Sosuke Aizen, after fighting a team of captains. His sword allows him the power to place his opponents in an absolute hypnosis, controlling the 5 senses and showing illusions. After tricking them all when they believed he hadn't been using it, one figures it out, and asks him how long he'd been using his illusion, and Aizen immediately responds with "Since when were you under the impression that I hadn't been using it?", despite fans themselves being unable to figure it out when in the timeline he'd activated it.

Since then, it started as a response to any questionable plot point, and eventually almost any situation. It's generally used by Bleach fans, but popular enough that most Anime and Manga fans recognize the meme.