Well, even I do have a few observation of species, made a few years before formal description, mostly invertebrates. But the problem is that all these cases are far from being equal. Many “well known”, charismatic and popular among phototgraphers species are still undescribed, as there are simply no living taxonomist working in that group. Or because species description in that group are too complex cause many species were badly described in past and a serious revision is needed. Many, if not the most, species being currently described were thought to be same species in the past.
I understand your point about rare species in well known group, but that is quite an exotic case…
Anyway, that might be an interesting statistics, if someone could dig it from Inat datatbase. That should not be very hard to get the list of observation with observation dates being earlier than year of description and sort them.