Has a gravitational microlensing event ever been predicted? If so, has it been observed?
In the pre-Gaia era, this was effectively impossible to do, since positions and proper motions weren't precise enough for a large enough sample of stars.  With the release of Gaia data, though, it has become an active area of research.
Bramich 2018 predicts 76 events, while Nielsen &amp; Bramich 2018 extend this to predict an additional 27 events using Pan-STaRRS data for very low mass stars as additional lensing objects against Gaia background stars.
Klüter et al. 2018 also predict 3914 events, and there may be other papers as well.
Note that many of the predicted events are for astrometric microlensing, where the position of the background star would be measurably shifted, rather than the usual photometric microlensing, where it gets brighter.  The latter requires closer alignment for a measurable event than the former does. But some of the events would yield measurable brightening as well.
McGill et al. 2020 suggest that many of the predictions are flawed, being due to either binary companions of the lens star (which would be co-moving and thus would not be lensed, since both stars are moving together) or to cases where there is a duplicate detection in the Gaia catalog (i.e. the candidate lens star and background star are both the same object).  They revise the overall list of predictions, but still retain many events.
As far as I can tell none of these events has been observed yet, but the predictions are there.