Half a large egg? crack it into a bowl, mix it with the fork and pour out half. I have to do that sometimes when i half recipes.
However, i would also use a small egg, if i had one, and would also use only the yolk, if it was somthing that needed a binder, like a potato croquette, that i didn't want to be dried out - or use the yolk in a baked good that i wanted moistness over rise (if it had leavening in it) or would use just the yolk if it were a recipe that i wanted to favor rise and lightness over moistness...
But yes, there are plenty of bad instructions, sometimes just typos, sometimes outright and outrageous contradictions, but i can discount those.
What i hate much more are recipes that are just too fussy and there is no need for it. Three cups plus one tbsp flour (in a bread recipe - a BREAD recipe) - come on, gimme a break. It's bread. It might need more and might need less flour, liquid, whatever, and you'll only know when you mix it.
Or beat the egg lightly with a fork and then add to the batter and beat. Do you really need to mix the egg first? certainly not.
or half a cup of chopped onion (come on, a little more, a little less is not going to matter - "one small onion, chopped" will do it.)
Or those recipes for the home cook that presume that everyone has a couple of people working for them to prep things and to wash up after them. Some test kitchen recipes are like this - they needlessly use extra equipment that needs to be washed when it could be avoided.
Italian recipes almost always have no description of technique, don;t list ingredients in the order in which they;ll be used, and (most annoyingly) will get halfway through the recipe before they tell you some step you should have done a few hours ago - for example, sautee the vegetables in oil, add water, and then add the beans that have previously soaked for 12 hours. Ok, you;re supposed to read through a recipe first, but a good recipe puts the various steps in order. Finally most italian cookbooks have horrendous indexes. You can;t find all the recipes using eggplant, you can only find those where "eggplant" is the first word in the name of the recipe (Ada Boni;s famous cookbook is like that) - you even need to know which kind of pasta she considers good with eggplant to find an eggplant pasta dish - e.g. penne with eggplant, or rigatoni with eggplant, etc.
In fact, give me a few minutes and i'll write a book on annoying cookbook quirks.
Oh, right, and how to store the other half egg? in a jar, so you can add it to the next omelette or for breading cutlets, or (as usually happens) so you find it in the back of the fridge two months later and throw it out, jar and all, not daring to open it. Or, more efficiently, just throw it out.