Generally it happens either when you neck up and fold some of the shoulder into the neck, or when you neck down a longer cartridge body so that the body becomes part of the neck when you reform. I get it out of Rigby or RUM brass when making something like a Jamison case (similiar to a WSM). As the body stretches, the thicker shoulder begins to fold into the neck and you get the donut.
If you don't seat the bullet all the way into the neck you have no concerns. However, if the bullet is seated deeper than the end of the neck, it can become a problem, especially if the neck is tighter than production chambers. For other cartridges where brass is commonly available, I've never found it to be a problem, even in custom chambers. It's fairly easy to check and in fact, if serious, you can feel the doughnut when you seat a flat based bullet. It's more difficult to feel with a boattail. The easiest check is to mike the neck after you seat the bullet both at the mouth and at the base of the neck where a doughnut might be located. If the difference is more that a couple of thou's, better be careful!
If you measure fired brass out of that chamber at the base of the base of the neck, you can measure the reload at the same location. If the reload measurement is larger than the fired brass measurement, DON'T fire that cartridge! If less, no problem.
