I always make sure bowl and beaters are clean, but I have had small amounts of yolk go into the whites that could not be removed. I have had the whites whip successfully in spite of the presence of the yolk. A small amount (1-2 tsp) of corn starch added with the sugar will prevent "weeping" of meringue in humid weather: I don't know if it will help prevent the meringue getting sticky or rubbery. It's best not to cover it. High humidity can cause meringue to fail, although I'm not a scientist and therefore can't tell you why. My grandmother was the pie expert and talked about humidity affecting meringue. It is very difficult, although not impossible to whip by hand. I gave my stand mixer away years ago because I rarely used it, preferring a fork for most things. I do however own an inexpensive hand mixer which I use for meringues and creams.