  A growing number of studies is being devoted to the identification of
plausible quantum properties of spacetime which might give rise to observably
large effects. The literature on this subject is now relatively large,
including studies in string theory, loop quantum gravity and noncommutative
geometry. It is useful to divide the various proposals into proposals involving
a systematic quantum-gravity effect (an effect that would shift the
main/average prediction for a given observable quantity) and proposals
involving a non-systematic quantum-gravity effect (an effect that would
introduce new fundamental uncertanties in some observable quantity). The case
of quantum-gravity-induced particle-production-threshold anomalies, a much
studied example of potentially observable quantum-gravity effect, is here used
as an example to illustrate the differences to be expected between systematic
and non-systematic effects.
