The only time I'm ever okay with the term "retard" is you are talking about cigarettes, and that's not to say that smokers are retarded. And that's because if you crush a tobacco leaf and roll some paper around it and light it, it will burn like a candle, so some chemicals are added as flame retardants. In that context, too, if you watch someone putting a cigarette in their mouth and drawing back, you'll see a glow like an ember as the paper and tobacco burn away into ash.

But I find calling an autistic person "retarded" offensive and especially because I still remember a kid at school saying to me, "Are you a bit retarded?"