Phule Me Twice by Robert Asprin & Peter J. Heck

Then, slowly, from a small arroyo a short distance away, a large form loomed up and began advancing toward them. In the night goggles, it stood out as a bright, throbbing presence, big as a man, moving directly toward the waiting sentries. “Gemini!” said Brick, and without waiting, she raised her weapon toward it.

“Hold it; that’s a person,” said Garbo, but she was too late: Brick had already depressed the firing stud.

In the night goggles, the stun ray appeared as a narrow cone of blue green light expanding toward the target. The cone enveloped the approaching figure, which was suddenly surrounded by a haze of reflection, and Brick lowered the muzzle of her weapon, waiting for the target to fall.

Except, this time, nothing happened.

The figure continued to advance on them. “Stop or I’ll shoot,” shouted Brick, now thoroughly befuddled by her stunner’s failure. “Put your hands up!” She aimed the stunner toward its head, even while she wondered what she was supposed to do now that the weapon wasn’t working. It must be broken; she’d seen it hit the target, no question at all. But if it wasn’t working, what was she going to do if the intruder attacked? Then it came to a stop, facing them, and raised its hands.

“Identify yourself!” she shouted. Behind her, Garbo was calling Mother, asking for backup.

“No reason to shoot,” came an eerily familiar voice. “I’m not armed. Can I come closer?”

“The captain!” Brick said, standing up to look more closely. It was impossible to make out facial features at this distance, especially with the odd color substitutions she saw in the night goggles; but the voice was undeniably Captain Jester’s.