This is a concern for me also. From what I read if it happens too much your chances of Herming goes up.
It does but what can you do? If you cant change it happening once why worry about it? If its day time put them in a sunny spot. Ive had to turn the lights off on my tent a few times over the months when there supposed to be on for several hours...mainteince man, parents coming over etc. Cant change the fact they came over so just have to deal with it.
I just bought a hi powered emergency light ($40). Power goes out light comes on. It obviously wont grow but could simulate a brief overcast. Power comes back on and emergency light recharges. I do need a time delay relay for the 5-10 min lag on the light wont cause problems.