Fridge Brilliance: Sonic loses his Boost, but gets back his Spin Dash. In Sonic Generations, Sonic taught his past self his Air Dash; conversely, meeting his past self has reminded Sonic of the Spin Dash!
When Dr. Eggman speaks to the Zeti about eating their black hearts during his little freak-out moment, it seems really out of place for him, but then I remembered something. Zomom spoke several times about eating Sonic and Eggman, and even Zeena mentioned eating Sonic at one point, so they are probably a race of cannibals. Dr. Eggman was speaking to them in terms they could understand. I'd say that qualifies as fridge brilliance, and maybe a little fridge horror too.
Among the new Wisps showing up in Lost World, there's Indigo Asteroid and Crimson Eagle, which in a sense replace Purple Frenzy and Violet Void from Colors. If you recall, the Frenzy and Void wisps were Nega-Wisps, corrupted by Eggman draining them of their Hyper-go-on energy. Due to the theming of the two - an animal and a space-themed object - as well as their general purpose, it can be assumed that the Crimson and Asteroid wisps are "purified" versions of the Nega-Wisps.
Fridge Horror: When the Deadly Six drain the life out of the planet, Amy and Knuckles lose their lives as well. Then you remember that not only is this affecting the whole planet, but is very likely affecting all of Sonic's friends there as well. Shadow, Rouge, Professor Pickle, Vanilla, Cream... who's to say the Gaias aren't affected as well?
What exactly did Master Zik teach the Deadly Six to make them the way they are now?