@Starlight Storm  
I liked the letters being used in Return of Harmony because it felt like the end of episode morals were setting them up instead of just being things I didn’t really care for at first.

@Solitair  
Plot contrivances can be pretty useful when done correctly, like the slide. (except of course the wrong term still bothers me)  
And the letters… they hurted that story for me.  
Deux Ex Machina is something that should be avoided as possible, but Plot Contrivances can be done as long as they don’t rely as copouts.

@Starlight Storm  
I’m used to thinking of those terms as things for writers to avoid, but I don’t think that this slide or the letters hurt their respective stories at all.

@FanOfMostEverything  
Actually, yeah. That’s pretty much correct.

@Starlight Storm  
So, all dei ex machinae are plot contrivances, but not all plot contrivances are dei ex machine, yes?

Pull the Lever Celestia!
 
CRASH!!
 
Wrong Lever!

http://m.youtube.com/watch?v=j0r-_Iermk4
 
Come on, I can’t be the only one who thought that.

Wrong. WRONG! Do you een know what Duex Ex Machina stands for Pinkie, and for that matter Price?  
Deux Ex Machina is, in a nutshell, divine intervetion that saves the day when everything seems lost, like Celestia sending the letters to Twilight CASUALLY the moment she got corrupted, and that CASUALLY happened to be the cure for Discord’s corruption.  
Now what happens here is a Plot Conveniance. Something that is relatively pulled out of their asses to get out of a situation easily and without much efford for the sake of keeping the story going. Learn the difference.