1) Some kind of dollar, global reserve crisis that rocks the global economy for a while. Spurred on by the US debt and projected near future large budget deficits due to welfare and entitlement spending.
2) Broader bond market. I think this is a well understood threat, and doesn't require much elaboration.
3) A specific junk corporate debt crisis. About half of the major corporate defaults in 2015 were US companies, which have levered up significantly on the back of the Fed's super easy money policies.
4) Another significant drop backwards in the asset bubbles the Fed just got done re-inflating. If the Fed's policies fail to provide enough to sustain those huge asset classes (real estate and equities primarily), then when they tip over it'll pull the US economy down into a protracted recession. The global economy is practically in a rolling recession as it is, if the US goes next it would cause a lot more global pain (including amplifying the problems in China and Japan, both big exporters to the US). There's a real debate to be had about whether the Fed can ever actually create a scenario in which the intentionally inflated assets can sustain (stand alone without Fed props), or if they have to always deflate backwards with a hefty penalty for the market manipulation / forced capital misallocation (which then prompts even more dramatic action at each turn, to try to re-inflate).