The main contextual change is that since late 2008 we have been, like all major central banks, increasingly resorting to unconventional measures to meet our mandate. Compared with conventional monetary policy (i.e. interest rates), these measures are less easy for the market to anticipate and their effects are less well understood.
This is because, first, unconventional measures can take many different forms and operate through different transmission channels. And second, there is no historical or statistical track record from which the market could deduce our policy reaction to a given set of circumstances. Moreover, while monetary policy always has side effects, the unprecedented nature of some unconventional measures makes those effects more uncertain.