Why does my coke can have a picture of flip flops on it?

Summer season coke?

I’ve also seen Coke cans with a sailboat on it. I vote ‘Summer’.

Like this?
I think it might be a vehicle for merchandising.

I think @chyna and @andrew are right about it being a summer can. There are several different designs for summertime (I’ve seen a grill, surfboard, and sunglasses).

Even minor changes to the branding of a product as ubiquitous as Coke are going to be noticed – and talked about. We’re all going to be checking our Coke cans for the rest of the summer!

Additionally, their marketing department has likely done testing to determine how many people (1 in 10, 1 in 100, 1 in 1000, etc) are likely to walk to the nearest Coke machine and spend 75 cents just to see what’s on that can, and can predict the amount of money their likely to net as a result of a basically effortless and temporary design tweak.

Or something like that.