Okay, maybe not a grammatical error...

...but these are expressions that are often misused or outright mangled. A lot.

"Well that's just a bold-face lie." It's actually a bald-face lie. That's why I put it in a bold-face font.

"I could care less." So you don't really care for something, but there's still some room to care a little less? What you probably mean to say is that you care so little about something that you "couldn't care less". Also (and I'm not sure where this came from) it's not "I couldn't care a less"

"Lord and behold". It's lo and behold. Yes, "Lo" is a word, albeit an archain one.

"There he was, butt naked". While I'm sure that all of him, including his butt, was exposed, he was in fact "buck naked."

"That's a mute point." When the subject of a debate is unimportant or purely academic, it's considered to be "moot" point.

"Happy New Years". Since we usually celebrate only one new year at a time, we're should be wishing everyone a "Happy New Year" (singular). This probably stems from our celebration of New Year's Eve, with an apostrophe preceding the "s" to indicate the possessive (i.e., the eve of the new year). So it's okay to say "See you at the party on New Year's", implying New Year's Eve, but not "See you in the New Years".