Its a problem i've experienced, 5G all grain batch and all the work that it entails down the sink

Basically the alpha acids get damaged by UV and form sulphurous compounds (rotten eggs, smelly farts, stink bombs you get the idea).
Its a problem that only seems to rear its head with extract and all grain since we tend to get our bitterness/alpha acids from fresh hops rather than isomerised alpha acids that they seem to use in kits.
I would guess on brew day that the beer is protected by its sheer quantity, even a light coloured lager looks considerably darker in the bucket so obviously the light can't travel that far through it, also the wort isn't usually exposed for that long (maybe 90 mins).
However when fermenting it can sit about for weeks, and when bottled the light strike/skunking can be even quicker due to the small volume of liquid which allows UV light to penetrate right through it.
Long story short, probably nothing to worry about if using kits.