I don't teach this for every takeoff, but certainly for conditions which are conducive to the formation of carb ice. In the particular case mentioned above, I initiated heat as the engine began to sputter. I experienced a power loss and shortly thereafter I regained power, albeit with roughness. Not at all uncommon. Water in the fuel, either in suspension or trapped, is not uncommon.
Entrained water settles out, and can take at least a half hour following fueling.
Likewise the same can happen during a preflight, or during temperature changes such as moving from a warm heated hangar to a cool hangar. You won't get all the moisture immediately, and if it's winter, you may not get it at all if the moisture is frozen. When it does thaw, it can create problems. Pilots fueling might not have found the contaminants right away, but eventually did. One pilot claimed an engine failure as a result.