Saw this today…on an electrician’s business card no less (!!):
“Let their be light.”
UGH!

Ok, here are my big ones, in no particular order:

1) Using an apostrophe where it just does not belong. Examples:
We have apple’s and peach’s
Smith and Smith, CPA’s
The house lost it’s roof

2) Dating a marketing piece so it can easily become outdated. Examples:
Let David’s 18 years’ of experience benefit you!
We have been in this business for a decade.

3) Just between you and I (should be you and me)

More later!

We’re asked for copywriting advice all the time. Feel free to read this piece we wrote on what’s crucial when it comes to using words to sell your products or service: http://www.getitinwriting.biz/info_detail.php?NewsArticleID=6

Have fun (and feel free to e-mail us your questions).