SUMMARY

there is no undo functionality for star rating (Bug 142753). This is especially dangerous when modifying the rating on a huge number  of images, which can easily happen unintentionally.

Example (this happens several times to me):
I rate all my holiday images (>1000). Images with > 3 stars should appear in a slideshow. For fine tuning (when a lot of work has already be done) I filter on >3 stars then select all and start presentation (alt sft F9). Stepping through the presentation I notice an image that should not appear and go back to modify it (rate with 0 stars). BUT still all >3 star images are selected! As a consequence the complete work of rating is lost!

As undo functionality seams to be very much effort, I would propose to do a warning, when changing ratings (or other tags) while all images are selected, e.g.: WARNING: this will modify xxxx images, do you really want do continue?
In my specific situation it might also be good to clear the selection after returning from presentation.