I've built about half a dozen wheels for my commuter bikes and never had any problems, but the last rear wheel broke some spokes.
Everything on the wheel was brand new, 32 spokes, had only done about 15 miles. I was pedaling hard, probably accelerating from 15mph up to 25 mph, and I broke a few spokes. I checked the wheel and it was okay to pedal slowly to my office as it was only a few minutes away, and I leaned on the bars to keep as much weight off the rear wheel as I could. Ended up breaking six (!) spokes by the time I got there.
All but one broke at the rim, and all but one were non-drive side leading spokes (i.e. the last set of spokes to be installed as per https://www.sheldonbrown.com/wheelbuild.html#leading).
There was obviously something wrong, maybe too much/too little tension, or a tension imbalance somewhere, but what could have caused just one set of spokes to fail so greatly? I would have expected drive side and trailing spokes to break first under stress.
Just so I don't make the same mistake again. In retrospect I should have checked the tension on the remaining spokes after the first two broke, that's what I did in the evening to get me home six spokes down and it was still fairly solid.