Pals of “True Blood” star ALEXANDER SKARSGARD say he’s notoriously bad at putting names to faces – even when the face is pretty and famous – but the Swedish-born actor blew the minds of buddies at an LA coffee shop when a sizzling hottie with super-short hair and even shorter shorts waltzed up and said “Hi,” gushed about loving the new season of “True Blood” – then flashed a big smile, purred that she’d see him around and split for the exit.

Said My SpyWitness: “Alex just nodded politely as she walked away until one pal blurted, ‘Dude! You don’t know who that WAS?’”

Alex looked blank until the guy sputtered: “That was MILEY CYRUS, you dumba**!”

Suddenly horrified he might have offended a fellow star, Alex ran to catch up before she exited, gushing, “It was good to see you, MILEY – please have a great day!”

Said the source: “Miley, smiling like she knew Alex was covering his butt, said, ‘It’s all right if you don’t remember who I am!’ – then reminded him they’d met THREE times at Hollywood club Beacher’s Madhouse.”