My early attempts at mayonnaise never failed. Then a few batches went wrong: I had got complacent. I had to relearn the tyro’s caution. One must add the oil very carefully at first, just a few drops at a time, and whisk vigorously to amalgamate it, before adding the next.
Pouring 150mls (1 egg) or 300mls (2 eggs) of oil at this rate would be very tedious. You do not have to. There comes a point at which you can be much more liberal. It is when the sauce stiffens: you can feel this process, and see it, because the sauce starts clinging to the whisk. Now, you can pour in generous glugs of oil before each whisking, and you are very unlikely to split the mixture.
Split mayonnaise, and what to do with it