The "dad reaction meme" drabble that sacrificusdux wrote has got me really thinking, on top of that parent anon meme I reblogged earlier.
It's pretty obvious that Levi has had some sort of conflict with his parents in the past, yeah? BUT THAT DRABBLE actually has me thinking of two things.
One could be that, he refers to Erwin as a significantly stronger father figure than his biological parents, and in his sleepy haze slipped up and called him "dad."
The other could be that his father used to wake him the same way, and and in his over exhausted state had a sudden flash back of his life before the Survery Corps or whatever happened to his parents.
eITHER WAY JUST KINDA HURTS THE SAME, YEAH? And in either case, something just points out to me that Levi's relationship with his father was less than ideal.