The club had promised to pay tribute to the 22 people who died in Monday's suicide attack during the clash against Arsenal at Wembley on Saturday.
The two teams and both sets of supporters had observed an impeccable minute's silence before the game kicked off, but viewers watching the match on TV were quick to spot something was amiss with Chelsea's kit.
"I can’t believe the Chelsea players are not wearing black armbands,” said one Twitter user. “How disrespectful.”
Kenya-based sports presenter Sean Cardovillis tweeted: “It has not been lost on fans around the world that Chelsea players, unlike Arsenal, are not wearing black armbands."
Chelsea players forgot to wear black armbands for the first half of their FA Cup final with Arsenal
The BBC's Dan Walker responded by tweeting: "For those asking… no one seems to know why Chelsea aren't wearing black armbands. Will try and find out."
The presenter duly got to the bottom of the mystery, tweeting that the club "forgot" to include the armbands with the kit.
"It's completely accidental," he said. "they will be on for the second half."
Fans quickly noticed Arsenal players were wearing black armbands but Chelsea were not
Chelsea players wore armbands for the second half after apparently forgetting to put them on
The Chelsea players did indeed emerge after half time sporting black armbands – but some social media users were still far from happy.
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"Forgetting’ to wear black armbands 5 days after worst terrorist attack in ten years – not a good look Chelsea," tweeted @EssieEmm
"How can anyone "forget" – disgraceful. No excuses" raged Jools_Orca.
The confusion over the black armbands was reportedly down to a players' disagreement over whether or not to wear their training tops for the pre-match ceremony.
The team ultimately came out of the tunnel in tracksuit tops but had apparently forgotten to put the armbands on underneath after the discussion.
The teams and fans observed an impeccable minute's silence before the match
All in all, it was a day to forget for the Blues, who went down 2-1 to the Gunners and had Victor Moses sent off.
Antonia Conte's men struggled to emulate the form that saw them win the Premier League two weeks ago, coming up against an inspired Arsenal side who deservedly secured a record-breaking 13th FA Cup win.
To compound Chelsea's misery, Moses was filmed throwing his black armband to the ground in frustration after receiving his marching orders for diving – sparking yet more ire on Twitter.