Baking – I forgot to add butter to the cupcakes but they were ok – why

Many frostings incorporate more than butter as the fat in the icing. I recently made ones using shortening, coconut butter, and coconut milk solids. These three are all fats that have higher melting points and are more reliable at higher temperatures in comparison to butter.

If you wish to retain as much of the butter mouthfeel as possible, you might consider switching to a buttercream mixture that splices in shortening or even margarine as both are more workable at room temperature. You might also try some of the other variations on buttercream.

Best practices for icing or frosting in any case are all over the web;

• This extensive article has a real gem to be wary of; keep the butter around 65-70 to ensure you aren't dipping too low or hot. There's plenty of additional information and a listing of strengths and weaknesses of the varieties.
• Here is a run-down of how to frost the cake itself.

• As alluded to in the comments, don't put meltable frosting/icing on a hot cake.
• If you are going to keep the frosting in the heat and sunlight, you are definitely going to want to add some shortening or coconut butter in there; that's why it's called Decorator's icing, they put shortening in it so it can hang out at summer weddings.

I will quote here the bible of cooking science, Harold McGees "On Food and Cooking":

"Dumpling doughs are minimally kneaded to maximize tenderness, and benefit from the inclusion of tiny air pockets, which provide lightness. [...] This tendency to rise with cooking is due to the expansion of the dough's air pockets, which fill with vaporized water as the dumpling interior approaches the boiling point and make the dough less dense than the surrounding water."

Following this, your dumpling must be a proper dumpling - dough that was only minimally kneaded, while altitude does not matter.

Why does this coincidence with being ready?

[...] the starch granules absorb water molecules, and swell and soften as the water molecules intrude and separate the starch molecules from each other. This granule softening [...] takes place in a temperature range that depends on the seed and the starch, but is in the region of 140-160°F/60-70°C. The tightly ordered clusters of amylose molecules require higher temperatures, more water, and more cooking time to be pulled and kept apart than do the looser clusters of amylopectin molecules.

So, altogether, the starch molecules do not absorb much more water as they are done, so the remaining water can vaporize and fill the air pockets, which makes the dumpling float then. Or, in other words, a floating dumpling is actually overcooked and so guaranteed to be ready (if the preconditions are met).

Does this rule ever fail?

Yes. Your dough needs to have sufficient air pockets for floating. Your dough needs to be made out of starch that is willing to absorb water. A dough made out of waxy potatoes has a fair chance to not float in time. This does match with German recipes for potato dumplings using waxy potatoes, that warn about the dumplings falling apart due to overcooking, if there is too much water in the pot (as it takes too much time for the dumpling to rise).