For example, I've personally done quite a bit of what's actually free soloing in cave exploration, but that style of climbing is done in confined spaces with rock on every side. You're much less dependent on individual holds as you'll usually be supporting yourself by pushing against the rock with your whole body, so it's a lot safer than most free soloing - even if you do fall you have a good chance of self arresting by, and if you can't you can at least slow your fall significantly and just slide to the bottom.