I thought it was the point where a teen age driver finally gets that 10th bump in the body work..

Deca = ten
Dent = You have to tell your parents/insurance company.

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Why do people deem it their job to correct spelling/usage/punctuation on the interwebz?

I really miss the days when people weren't so 'incapacitated' over the use of a word/phrase/spelling.

People that point out someone's error usually are way too blind to see their own shortcomings.