Trend is a noun. It means “A general direction in which something is developing or changing”. You can’t add “ing” to the end of a noun and pretend it’s a verb. So the non-word “trending” is just that; a non-word. Now Microsoft is computering this in Windows 8, with a “trending” tile on the start screen (see how stupid it is adding “ing” to a noun?)
Have you seen this one: “For all intensive purposes”? Oh, the pain! It should be “For all intents and purposes”.
I can’t believe people do this stuff. Doesn’t anyone ever say these things out loud and then think to themselves “well, that makes no sense, maybe I got it wrong”? Apparently not.
How about “No holes barred”? That one started as an amusing twist on “No holds barred”. The original phrase was from wrestling and meant that any “hold” was fair game – none were prohibited, or “barred”. The mangled phrase was coined by the porn industry. So the moral of the story is: If you are using a catch phrase, know what it means and use it correctly!