In my personal Head-Cannon, the Killing Curse that got Moody, simply hit his magic eye, breaking it, but allowing him to survive.
"Mad-Eye's dead." Bills voice seemed to echo in the night air. Mad-Eye Moody? Dead? The weather-beaten, cursed and scarred but never out of the fight Mad-Eye, who was the consummate survivor? How could Moody be dead?
"Voldemort caught up to them, and 'Dung disapparated. The Killing Curse hit him in the face," the curse-breaker elaborated reluctantly in the stunned silence.
"Do you think 'Dung?-" Arthur asked reluctantly, even after Wormtail, then Snape, it went against everyone's grain to accuse another of betrayal.
"No, they weren't expecting the decoys, and "Dung came up with that bit of skulduggery." Remus said, slowly. There was a long, shocky silence.
"Well, Mad-Eye wouldn't want us to just sit here, thinking about him." George said.
"Indeed, he'd be yelling at us for dropping our guard. CONSTANT VIGILANCE!" Fred barked, in an eerily adept impersonation, making Ginny slap him. Weak laughter broke the tension.
"Right," Arthur said, and went to the mantle, opening a bottle of fire whiskey. He conjured thirteen shot glasses and filled them-
"Wouldn't happen to have another one of those for me?" Came an unmistakable raspy voice from the door.
Instantly a multitude of hexes flew, and thirteen wands were pointing at the impossible vision. In the door of the Burrow stood a bloody, battered Mad-Eye, clotting smoking from barely missed hexes in several places, and missing the distinctive eye that gave him his nickname, nodding approvingly as the spells bounced off a shield charm.
"Excellent reflexes Potter, Granger, Bill. You got off yer hexes before the others got their wands out," he growled. No one lowered their wands at all.
"What were the first words you ever said to me?" Harry snapped, his wand tip glowing ominously.
"We were in the Hogwarts hospital wing, and Poppy had just finished driving us mad with her hovering. I told you that if being fussed over annoyed you, you should avoid healers at all costs. When was the first time you saw the me?"
"When Dumbledore opened the seventh compartment of your trunk, and had me pass him the imposter's cloak." The teen said evenly, slowly lowering his wand.
"Granger! The first spell I taught you!" Hermione sniffed.
"You never taught me a single spell, but I taught myself the disillusionment charm after watching you use it." She'd lowered her wand at the same time as Harry, but no one missed the large cloud of canaries ready to resume the attack, circling above her head. Bill was staring in suspicion and shock at the man, while Kingsley and Tonks had their wands inches from his face.
"How did you survive being hit by the killing curse, Alastor?" He growled, the sound being a startling reminder that the easy-going man had some wolfish traits after his run-in with Greyback.
"Well it didn't hit me, now did it? Hit th' magic eye, blew it t' shards, and bounced off towards the Death Eater scum, now didn't it? Knocked me clean off me broom, but that is what charms are for, isn't it?" Looking closer, the watchers realized that the empty socket was covered in cuts and shards of glass were embedded in the ex-auror's flesh.
"He smells like Mad-Eye, and not poly-juice. Harry, Hermione, are you convinced?" Remus asked, making people jump, having forgotten that the quiet werewolf was in the room. He didn't often make use of his enhanced senses, but the green-eyed teen was glad that someone else was reinforcing his gut-feeling that it was indeed Mad-Eye. He glanced at Hermione, and Ron, who both gave tiny nods.
"George, budge over for another injured warrior, would you?" Harry said by way of answer, and immediately Molly began fussing over Moody, who was trying, quite unsuccessfully, to brush her off. Within seconds he was on the sofa, grumbling as she removed the bits of enchanted glass, and dropping essence of Dittany on the cuts.
"Same as George, I'm afraid, the curse won't let me heal it up any better, Alastor." Molly said regretfully, standing back, and running her hands through said twin's hair.
"Mum! You're going to make me look like a ginger Harry!" George protested, but didn't pull away.
"Got yer ear cursed off, did ye lad?"
"Snape." Remus growled, looking guiltily at the injured Weasley.
"Moony, old man, don't look like that," Fred began,
"We're glad you managed to get me back," George continued,
"Mostly intact, even if he's Saintly."
"Don't know if you noticed, old bean, but Snake-face was coming up behind the greasy traitorous git." THe injured twin said with a shudder.
"You saved my boy," Molly said, and promptly burst into tears all-over the embarrassed Remus, the werewolf's wife mouthing 'I told you so' at him over the distraught mother's shoulder.
"Lupin always blames himself too much." Moody said, earning himself a poke with a wand from a startled Hermione, and a 'Bloody HELL!' from Ron, while Harry simply snorted.
"We need to work on your situational awareness, guys."
"Not our fault that Moody's way to blood sneaky for a guy with a fake leg!" Ron protested.
"Harry's right Ron, not noticing things is likely to get us killed soon." Harry knew she was referring to the upcoming Horcrux hunt.
"Ye kids ought t' learn subtlety, if ye are going off t' hunt Horcruxes." The ex-auror said, voice low enough that only the three of them could hear. This time all three teen jumped and swore, under their breaths.
"Did ye really think Albus would leave it all up t' chance and three kids who aren't even fully qualified yet?" Moody snorted. "Not t' mention he's forgotten that I was th' one t' teach him how t' pump people for information. For a man obsessed with keepin' secrets, and pryin' into other people's heads, he was remarkably poor at keepin' me out. Not one damn thin' in his maudlin farewell letter surprised me, and that flamin' chicken won't give me a moment's rest if I'm not guardin' Potter." Suddenly the lop-sided smile looked remarkably evil. "Now, I have a free hand in trainin' you lot, just like I wanted from th' start." With that, he stumped off towards Kingsley, who looked like he wanted to escape the emotional scene, but was trapped by the fact that the wailing Molly was right by the door. Harry turned to look at his best friends.
"Why do I have the feeling that we'll be wishing we were on our own, rather than with him, before three days are out?" The bespectacled teen groaned. The other two just looked at him mutely.
Should I expand this into a full story?