Swords of The Coast lists the flying ability for the Winged Tiefling. One player in a group I DM created a "Tiefling (Infernal) Warlock (Undying)" which he has been very specific and detailed about since he created him. This past weekend in a critical part of a dungeon, he was flipping through the Swords of The Coast book, which he had just bought earlier that same day, and told me the book said his Tiefling had bat wings and could fly.
The PHB lists no flying speed for Infernal Tiefling so I assume the flying ability is an option for a variant version of (Infernal) Tiefling. I want players to have the characters they dream about when they build their characters, but the Warlock player didn't know about flying before - I'm almost positive he didn't because he tried to climb off a 25 foot pillar and fell two sessions ago. He took minor fall damage and no one said anything about flying.
I guess I'm trying to figure out what to do about this. I don't want the players discovering new skills and introducing them unexpectedly after level 1, much less in the middle of a battle.