junior moment
n. A momentary lapse into immaturity; nervousness or folly caused by youth or inexperience.

Example Citations:
Senior moments are not just for seniors. Did Hillary have a middle-aged moment about the sniper attack in Bosnia that never was? Did Barack have a junior moment when he wrote about reading a Life magazine article on a man who tried to lighten his black skin? An article that never ran.
—Ellen Goodman, "McCain's 'senior moment' opens up a new -ism," Pittsburgh Post-Gazette, March 28, 2008

Of course, such things shouldn't be taken too seriously. Most people, surely, want to get old disgracefully, to go not "gentle into that good night". Most would agree that age, its frustrations and illnesses, even, can be treated with a dose of humour. I look forward to the Melbourne Comedy Festival's feature, Alzheimer's the Musical. A night to remember.

Most can be forgiven for sharing a laugh at themselves, their problems, senior, intermediate, junior moments.
—John Huxley, "Feel free to grow old disgracefully, but hold the bellyaching," Sydney Morning Herald, February 24, 2007

Earliest Citation:
House Fiscal Analyst Elise Read, after stumbling a few times trying to explain the tobacco settlement in a room packed with dozens of House members, pleaded for mercy. "I'm nervous," she said. "I can do this at the committee level, but I'm not used to this many people." "Don't worry," said House Speaker Pro Tem Peppi Bruneau, R-New Orleans. "We all have our senior moments. You're just having a junior moment."
—"Briefing book," Times-Picayune, February 20, 2000

Notes:
This phrase is, of course, a younger version of the now-famous phrase senior moment — a momentary lapse in memory, particularly one experienced by a senior citizen — which first appeared in the language (if I remember correctly) in 1996.

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