In both of these incidents the symptoms showed up at times I certainly wasn't expecting to find ice growing in the throat of my carburetor, so I was a bit slow to react properly by applying heat. Because of my policy of using full heat when I have the slightest suspicion that there might be ice around, these are really the only times I've encountered carb ice. This is what experience has taught me. This was an interesting and informative talk, even for CFI's. Mixing any two above should give clear sample.
Higher octane slows combustion, avoids knocking.
Hold fuel nozzle against side of filler tube to avoid spark. Dry gas, once a year, to get rid of water in creases or nooks. Reverse for starting a climb. Aircraft performance tables are based on proper leaning. Keep a timer with alarm going.