In my case, the 5% swings are simply "noise", where nothing is working for a week or two. Happens more than I like, but I don't think it can be eliminated.
The 15% was two nasty currency trades with monster overnight gaps. Took the losses over a two day period. Obviously, I had underestimated position risk as I would prefer to NOT lose close to 10% per position. I usually size each system such that i'm looking at a simulated worst-case DD of ~ 10% across the system portfolio.
Edit: a "perfect storm" for me, would probably be close to 30% DD (based on simulations).