Mustafi has been the subject of ridicule by some Arsenal fans in recent seasons.
The central defender is disliked by sections of the Emirates faithful following some performances under Unai Emery last term.
Mustafi has been linked with a move away from Arsenal this summer.
But the German has insisted he wants to stay at the Gunners beyond the August 8 transfer deadline.
Mustafi was handed the captain’s armband during Arsenal’s friendly clash with Colorado Rapids last night, sparking surprise from some fans.
The former Valencia man pulled the armband up his sleeve – but eagle-eyed fans noticed something wasn’t quite right.
Mustafi had accidentally put the armband on upside down.
And Arsenal fans took to Twitter to mock the gaffe during the 3-0 win.
“Mustafi can’t even put the armband on correctly ffs Arsenal,” said one.
“Only Mustafi can be given captain for a preseason game and take his chance by putting on the armband upside down,” one wrote.
“Mustafi is so used to ending up face down on the floor, he’s actually preempted it by wearing the captain’s armband upside down so it looks right on TV,” joked another.
“Can’t even were the armband the right way up,” added a fourth.
“Just noticed Mustafi can’t even wear the armband properly, he wore it upside down,” said another.
Mustafi looks set to stay at Arsenal this season.
But Emery wants more new arrivals in the coming weeks.
“The club is working for new signings and we are speaking every day with the club and with Edu about that,” he said after the win.
“We have very good young players but we need someone to help us.
“But I prefer to work with patience and calm to take the best decisions.
“If for this season we need to wait for some more time, I prefer to do that.”