I found this on a kiltmaker's website, as an advertisement for their wares.
The kilts don't look awful (although a bit long), and the sash things are silly, but I thought the rest of the look is what makes this a "Don't".

Don't wear flashes with your short socks when wearing a kilt.
Don't wear ... whatever socks the groom is wearing with his athletic shoes.
Don't wear athletic shoes to get married, unless there is a chance you'll have to run away before the ceremony's over.
And lastly, please don't wear official "Highlander" - style katanas, unless you're in one of the films.