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Popular thought is that the formation of a pellicle is not indicative of the quality of the sour beer that is being produced; it is only an indication that oxygen has entered the fermentation vessel and that the microbes are reacting to that exposure. One myth about pellicles is that the sour beer will be ready to package once the pellicle falls out; there is actually no correlation between the maturity of the beer and pellicle formation or dissipation. Some sour beers never form pellicles, and turn out fine as well, so the formation of a pellicle has no correlation with the quality of the beer.
Another less common but occasionally discussed myth about pellicles is that sunlight might cause them. There is no evidence to suggest that sunlight has an effect on pellicle growth in beer. A more probable explanation for pellicles appearing in fermenters that have had sunlight on them is that when the sunlight hits the fermenter, the fermenter warms up. As it cools, a vacuum is created and air is sucked into the fermenter. Mark Trent demonstrates how readily air can suck into a fermenter when the fermenter is cooled (temperature shifts in general and atmospheric pressure changes can cause this):
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