Either because the change is so small or because you know about the flashlight - or a combination of both - you manage to reach an understanding. You still remember the prank, but now you know it's a planted memory to explain the clothing transformation. Still, you play along to avoid suspicion.
"You weren't supposed to tell him," you tell Adam.
"Uh, I think he knows," Adam says sarcastically. "They're your sister's clothes, Eric. She helped us prank you."
"What are you guys talking about?" demanded Eric. "These were my clothes until a minute ago."
"Oh, suuure," you say. "I'll bet that's your flashlight, too." Before Eric can realize the significance of that, you snatch the flashlight out of his hand and pocket it. You think you see a brief glimpse of suspicion in his eyes but you quickly move the conversation to a new topic.