Once is an incident.
2.20, What Is and What Could Never Be
Twice is a coincidence.
6.22, The Man Who Knew Too Much
Three times...
8.09, Citizen Fang
...is a pattern.
rocksalter, on Tumblr, pointed out the one from 6.22, where Sam is comatose in Bobby's panic room for most of the episode. The screencap is from the beginning of the episode, before he learns that he's in a dream world. Before.
ETA 2: I just realized that both of the earlier season pics are from episodes where SOMEBODY HAS TAMPERED WITH THEIR MINDS.
Another thing we've all noticed is how much Sam's flashback of Amelia and the picnic with the birthday cake resembles Lisa's picnic for Dean in "Dream a Little Dream of Me," but look at this:
Open picnic basket in the bottom left quadrant, part of which is out of frame. Object immediately to the right of the picnic basket. Wine glass. Lowcut sundress. Hairstyle. Position on the blanket. Something obscuring the feet. Strappy sandals. The tall leafy vegetation in the background.
That is not an accident, that is a point for point recreation of Dean's picnic dream.
ETA: I actually forgot about this, but it fits the pattern better than the Madonna reference:
One is an incident.
Two is a coincidence.
Three is a pattern.
Something is very wrong with Sam's memories...
And then there's the Madonna reference that
camille_is_here pointed out in the comments of
this post: