> I went through my latest "developer challenge" experience. ~4 hours of work rejected because of a single bug.

Consider yourself lucky. Do you really want to work with such people who don't value your time and are so nit-picks?

Whenever, an employer asks me to do a challenge, I ask them how long do they think the challenge will take. I double the given estimate and if it is more than an hour or two, I ask to be paid. Most employer decline and I decline to further pursue. Simply, if they don't value my time now, they are not going to value my time after they hire me definitely. I maintain a list of such companies so that I can avoid them in the future. I also don't use or recommend their products and as a place to work to anyone.