BtVs by Whedon & M.E.
Xander watched as Buffy started fighting the blonde woman who was quickly shaping up to be the year's Big Bad. He and Willow had snuck down to the Reptile House, in order to watch the fight.
Glory had already started by dropping a cobra and her piece of a bloodstone into a ceremonial urn and chanting some words along with one of her minions, but, she was distracted mid-ritual by the Slayer's attack.
Seeing that Buffy was overmatched, Xander ran forward and kicked over the urn, which shattered.
From a safe distance, Willow telekinetically formed some of the urn's fragments into a rough scoop and used them to push the hissing snake in the direction of Glory's minion. The smallish demon screamed and backed into a corner.
Hearing the interruption, Glory turned in time to see Xander pick up the newly-energized bloodstone.
"What, bringing your friends into a fight?" Glory sneered and, turning at a precise angle, backhanded Buffy with all the strength she could muster.
Buffy went flying directly into Xander. Together they smashed backwards through the glass of a cage. Xander's hand was still holding the stone fragment and, as the edges dug into his skin, he landed on top of one of the cage's residents, which bit him in self-defence.
A bright flash of light filled the room as Buffy scrambled to her feet.
Turning around, she saw that Xander had been turned into a giant were-chameleon, just before the large creature faded from view...
Episode: The Weight of the World
Doc had been reading when Spike and an unknown walked into his room. He blinked at them owlishly.
"Oh, don't mind Xander," Spike said, gesturing at the man behind him. "He only gets uncontrolled during the full moon."
Doc examined Xander, noting how his skin was a pale grayish-pink and how his eyes extended slightly from his head into cone shapes. One of Xander's eyes was pointing towards Doc, while the other was examining the fireplace.
Having a possibly new species of demon in the room that he, with all his knowledge, didn't recognize was so unnerving to Doc that his pretense of a good-natured and helpful fellow wore thin a bit more quickly then it might have otherwise.
When Spike identified the box with the scrolls Doc had been concealing, Doc decided to disable the unknown first and, with an immense feat of speed and agility, he put Xander in a headlock and put a sword to his throat.
Spike rushed forward only to be met by Doc's secret weapon, an immense tongue that shot out of his mouth with enough force to slam the vampire against the wall.
Xander took advantage of the moment to work his way free and run to the opposite side of the room.
Doc spun in place to see the young man staring at him calmly, both conical eyes facing the same direction for once.
Doc opened his mouth, only to have Xander's longer and faster tongue hit the center of his forehead before the demon could get a shot off.
Doc crumpled in a limp pile toward the floor, the front of his skull crushed.
"Come on," Xander said. "Everyone knows chameleons have the fastest tongues..."
Spike groaned and pulled himself to his feet.
"Well," Xander said as he moved towards the box with the scrolls they'd come for. "That almost makes up for turning into a giant unintelligent reptile who has to be kept away from babies, housepets and Dawn, because her green glow is just so tasty
"What're you going on about," Spike asked, brushing himself off. "You caved in the bloke's brain and I didn't even get a turn."
"Oh well," Xander said, gesturing dismissively at Doc's broken body. "Just take the sword and, I don't know, behead him or something..."