  The role of measurement induced disturbance in weak measurements is of
central importance for the interpretation of the weak value. Uncontrolled
disturbance can interfere with the postselection process and make the weak
value dependent on the details of the measurement process. Here we develop the
concept of a generalized weak measurement for classical and quantum mechanics.
The two cases appear remarkably similar, but we point out some important
differences. A priori it is not clear what the correct notion of disturbance
should be in the context of weak measurements. We consider three different
notions and get three different results: (1) For a `strong' definition of
disturbance, we find that weak measurements are disturbing. (2) For a weaker
definition we find that a general class of weak measurements are
non-disturbing, but that one gets weak values which depend on the measurement
process. (3) Finally, with respect to an operational definition of the `degree
of disturbance', we find that the AAV weak measurements are the least
disturbing, but that the disturbance is always non-zero.
