Ok so I am not really sure if this counts as a flashback since its like not an actual flashback on the show? I don't know if that make sense but whateves
So it was in season 6 episode 15 "Across the Sea" and it was when we found out that the black and white pieces were from Jacob and MIB, I honestly loved this part because it shows that the creators knew what was gonna happen the entire time:
If you don't think that counts as a flashback/forward/sideways then my favorite would be the FINALE OBVIOUSLY.
I thought it was insane... Flash sideways I thought were just for another view I didn't know it was gonna be a major part of the show but I loved the finale:
and for those who didn't like the finale here are the alternate versions LMFAO: