Genuine one-off costs exist. The problem is the company that reports one every single year, in which case the charges are simply a cost of doing business that never appears in the non-gaap numbers investors quote.
A practical test: add up the last five years of "one-time" charges and compare the total with five years of adjusted profit. If it is a meaningful fraction, treat the charges as recurring.
Example: Northwind takes a $28M restructuring charge this year and took $19M, $31M and $22M in the three years before. Averaging $25M a year against $120M of operating income, this is a running cost, not an exception.
Related: impairment