On: 0 degrees celsius + 0 degrees celsius -

[jcdreads]

It's meaningless to add temperatures in any circumstance. You can average temperatures, and you can add amounts of heat energy, but it is semantic garbage to try to add temperatures.

I'd say what it's doing--converting to an absolute scale then adding--is an interesting bug.

[alextgordon → jcdreads]

Yes, temperature addition is garbage, but the addition of an interval to a temperature is not. Consider:

    "It was 20 °C yesterday and someone told me it's 10 °C
     hotter today. What temperature is it?"

The difficulty comes when you try to write that out using addition. This is OK

    "10°C + 20°C"

because all of the units share the same null point so there is an implicit null point at 273.15 K.

This is not OK:

    "293.15 K + 10 °C"

because the null point could be either 0 K or 273.15 K.

If I were google I'd just show two answers in this case:

    "10 °C hotter than 293.15 K = 303.15 K
    "293.15 K hotter than 10 °C = 303.15 °C"

[georgecmu → jcdreads]

Doesn't averaging temperatures require addiion?