I received a copy of a home inspection report on a brand new house we just wired. I've never heard of it either. The report states:
"Arc fault breakers "stacked" in the panel - they should be separated by typical breakers"

Then again, another "problem" found was:
"Master bedroom outlets at bed location on arc faults"

The house is big money, a little south of 3 million, and I assume the report is being used as a negotiating tool.