If you are talking in general about inadvertent upshifts, then the rider must be doing something very wrong with the shifter to be upshifting w/o wanting to.

If you are talking instead about inadvertent upshifts while on the large cog only, then something is wrong with the shifters becasue the upshifting technique/feel SHOULD be identical across all cogs (except the smallest, of course)

If you are talking about inadvertent upshifts while trying to downshift after running out of bigger cogs...see my post above