Over the years, there have been a number of well-intentioned though controversial ads that alluded to the events of September 11th. Here are a few examples from last year.
There was public outcry over this ad from Tumbledown Trails Golf Course in Wisconsin:

Also last year, consumers didn't take kindly to AT&T using the Twin Towers to remind them that AT&T sells smartphones:

The LA Lakers had to delete this tweet after fans mistakenly thought the photo was meant to be a humorous jab at Kobe Bryant's Afro:

The Starkville (Mississippi) Daily News ran this promotion last year:

And Marriott got into hot water when one of its hotels made guests this offer:

Last is this 2007 ad for a piano concert held in memory of 9/11 victims. Personally, I think this allusion to 9/11 is appropriate and and not offensive. Do you agree?

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I hemmed and hawed about posting this yesterday. Finally decided to take the advice I'd give a client: If you're that worried you might offend your audience, don't do it.