I used to "set and forget" quail eggs until lockdown but I had an egg explode and I never EVER want to deal with that again!No visible fertility signs at day zero without cracking one open (that's actually what I do with the ones that don't pass my inspection). They'll have the same bull's eye like a fertile chicken egg. "Go bad" as in contamination, they stink and leak of you don't catch them in time. I candle day 4 to check for veining, you can't always see through well but developing eggs often have a spot that had a pink cast. I sniff the air vent daily as well, just to be safe. I have had 3 contaminated eggs this year even with being selective, been incubating a lot of shipped eggs for new bloodlines and colors.