  Half a century ago a local and (seemingly) causally consistent implementation
of the projection postulate was formulated for local projectors in Quantum
Field Theory (QFT) by utilising the basic property that spacelike local
observables commute. This was not the end of the story for whether projective,
or ideal measurements in QFT respect causality. In particular, the causal
consistency of ideal measurements was brought into question by Sorkin 20 years
later using a scenario previously overlooked. Sorkin's example, however,
involved a non-local operator, and thus the question remained whether ideal
measurements of local operators are causally consistent, and hence whether they
are physically realisable. Considering both continuum and discrete spacetimes
such as causal sets, we focus on the basic local observables of real scalar
field theory -- smeared field operators -- and show that the corresponding
ideal measurements violate causality, and are thus impossible to realise in
practice. We show this using a causality condition derived for a general class
of update maps for smeared fields that includes unitary kicks, ideal
measurements, and approximations to them such as weak measurements. We discuss
the various assumptions that go into our result. Of note is an assumption that
Sorkin's scenario can actually be constructed in the given spacetime setup.
This assumption can be evaded in certain special cases in the continuum, and in
a particularly natural way in Causal Set Theory. In such cases one can then
freely use the projection postulate in a causally consistent manner. In light
of the generic acausality of ideal measurements, we also present examples of
local update maps that offer causality-respecting alternatives to the
projection postulate as an operationalist description of measurement in QFT.
