Joe Biden's social media team got some 'splainin' to do after posting a shot of the candidate with Barack Obama ... and placing a giant letter "N" over the former Prez.

It was a clever idea ... using multiple pics of Biden to spell out "Biden President" on his Instagram page -- but the HUGE FAIL came when someone on Biden's staff didn't realize where the "N" at the end of Biden would land.

The faux pas is being called out by users who think it's time Biden get a new social media manager -- y'know ... one who realizes Instagram users get individual posts in their feeds ... not 6 at once!

As one user put it, "This is the only post that showed up in my feed, it looks racist af."

Look ... Biden and Obama's bromance is well documented, so no one could really think this was intentional -- and certainly not anything Biden himself approved.

Still, it's a bad look.

For what it's worth, the "B" in Biden's name landed on a pic of him with his wife, Jill.