In science fiction fandom, the legendary example was Lee Hoffman, active fanzine fan and writer of Westerns as well as S.F.  Here's part of Wilson Tucker's classic account of learning the truth, in Room 770 of the hotel where the 1951 World Science Fiction Convention was being held. He has just stepped out of the shower:

I wrapped a towel around my middle, began searching my luggage for a deck of cards, and yelled a bored invitation to enter.

Three strangers trooped in wearing abashed grins, a girl and two men. The girl looked as if she were desperately searching for better company than the characters trailing her. I silently sympathized, and stared at the trio, the meanwhile dripping soap and water on the rug. The two gentlemen stared at the towel and giggled, while the girl looked at the puddle on the rug.

"Hello," one character said.
"Hello," another character said.
"Hello," the girl echoed.

Sadly, I shook my head. The same old wornout greetings.

"We're faaaaaaans" the tallest character announced proudly.

"The hell you say!" I shot back, astounded.

"Yep." He was wearing a white T-shirt on which had been printed, I AM SHELBY VICK. Turning to face me, he asked: "Know who I am?"

I gazed at the T-shirt. "Bela Lugosi?"

He waggled his head, vaguely disappointed.

"Richard Shaver," I guessed again, "Claude Degler, Ray Palmer?"

"I an Shelby Vick!" he exclaimed then in clear, ringing tones.

"The hell you say!" I shot back, astounded.

I-am-Shelby-Vick then flicked a finger at his two conspirators. "You know Lee Hoffman, of course?"

Of course. I threw a bored glance at the remaining character and yawned, "Hello, Lee."

"No, no!" contradicted I-am-Shelby-Vick. "Not him ... HER!"

Mustering what dignity I retained, I picked up my towel from the floor and stalked into the bathroom, flanging shut the door.