the more closely you look, the more obvious it becomes that this is the rule rather than the exception. I would be somewhat surprised if children have functional mutations rampant between neurons, and I suspect that some fraction of this is artifactual. But I have no doubt (does anyone?) that some degree of somatic mosaicism is the rule. About the only cells that tend to hang around much longer than neurons are blood stem cells, and as soon as you look closely at those, it's all but unavoidable.