[bluedevilzn]

This is due to ad blindness.

The Previa ads were always there but that was the first time you noticed it because it was on your mind.

[antiraza]

Alternatively, the Previas were always there, and they developed the memory of seeing it after having been exposed to the ad.

(To be clear, I'm not being facetious -- this is a viable possibility)

[monkpit]

Baader-meinhof phenomenon