Spoiler from Uncanny Avengers #4!
In the Decimation story arc, the overwhelming majority of mutants lost their powers due to the magic done by Scarlet Witch. This evidently included Quicksilver (Pietro Maximoff), her own brother.
But how he did he lose his power when the latest retcon reveals that he isn't actually a mutant?
Why was Quicksilver affected when other non-mutants weren't? Spider-Man and the Inhumans retained their powers, so why not Quicksilver?