While it's not an exact match (by ANY MEANS), I just realized how creepily close my fanfic The End of an Age I wrote (for a contest on here) was to the actual magic reveal in the show (and I SWEAR I had no idea it would end that way -except for Merlin being Bedivere, which was just a GUESS on my part early on- as I wrote this before enough spoilers to even so much as hint at the reveal were released)!
Merlin comes to the battlefield first as Dragoon, though Arthur doesn't know it's him, and then as himself where he helps an already fatally wounded Arthur (by Mordred, who was dead by Arthur's counterstrike) get away (and the battle was already pretty much won/over with) and then reveals he has magic.
Given, there are several differences, including that Merlin has a little more time to tell Arthur EVERY little detail in my fic, whereas the show was more show than tell, naturally. The bigger ones being Morgana is more or less redeemed, Guinevere gets to say a last farewell to her husband rather than just getting his ring back and being told he's dead, and more than just Freya's arm appears (Lancelot and Elyan had a cameo in my fanfic, as well). Also, following the series four, "Is this really what you wanted, Morgana?" bit my fanfic had more Merlin/Morgana interaction than a mere, "It's my fault, I'm sorry," -runs her through with a sword.
But the things that WERE the same (Merlin throwing the sword, the reveal and Arthur's general reaction, and Freya's arm) just occurred to me and I thought it was neat in a creepy, "how about that!" way.
Here's the fic itself for those of you who haven't read it and thus don't know what I'm talking about: http://www.fanfiction.net/s/8717478/1/The-End-Of-An-Age