The "plethora effect" is when kids use words in college application or SAT essays that sound good but they don't really know the meaning of. Sometimes they'll accidentally use thme as words that sound similar but have completely different meanings (for example, accidentally saying homeliness instead of homelessness) or sometimes they'll just be totally wrong. It was given the name a few years ago when someone decided that "plethora" was a 'good SAT word' and everyone started using it in college essays, much to the chagrin of the people who actually read the things. It's basically a result of kids prepping for the SAT and college process too much. (Starting SAT review books in middle school, taking tons of Princeton Review classes, etc.) Can sometimes be helped with the use of spellcheck.
Admissions person 1: "Look at this, this girl's essay is about how 'fragile and venerable' she was as a middle schooler, and how much she's grown."
Admissions person 2: "That's the plethora effect for you..."