Stressing a plant.

Generally, when growers talk about 'stressing' an mj plant, they're referring to factors that would tend to turn the plant into an hermaphrodite, with both male and female flowers. Extreme temps, humidity, etc. can cause a female plant to think that it's going to die soon and start to grow pollen sacks in a last ditch effort to pollinate itself and reproduce. But, light leaks during flowering are the main cause of stress than induce hermies.

 

^What John said. Stress can look like nothing until hermie budding shows. It can also look like nothing but results in slow/stunted growth. Or, it can make the plant susceptible to disease, less tolerant of pH fluctuations that affect nute uptake, etc.

 

The problem with answering your question about light leaks is, what's "a small amount"? You would see that differently than me, who would see it differently than the next guy. Certainly there is an infinitesimal amount of light that would not harm the plant, but nobody knows if your idea of a small amount is above or below that line. And then people would always want to be pushing that line -- well if that amount is OK then a tiny bit more wouldn't matter, would it? So, best rule is total darkness, zero light leaks, zero interruptions to the dark cycle.

FYI the moon gives off very little light in actuality, it looks very bright to us because of the contrast against the blackness of space, and keeping in mind that humans perceive light, we don't experience it and use it directly the way plants do.