In the first season of Attack on Titan, Eren remember something his dad told him about the cellar under their house, when he saw the key, and he transform into a Titan and stopped a canon fire.
We first saw that key with Eren's dad. While he was leaving he said to Eren something like "Be good and you will get this when I get back". Then when the Titans attacked and Eren went to Wall Rose we see the Key in his neck.
I wondered how it got there, but the flashback scene suggest that his dad erases his memory.
But I was wondering, the key was with Eren for the past 5 or so years. If a look at the key would make him remember everything, why didn't it happen during that time?
I am now half way to season 1, and haven't read the manga so if spoilers are there please put a spoiler tag