Several.
I backed a few 5e campaigns for physical rewards and had stopped running 5e by the time they arrived because of delays. Recently I received the books I pledged for a PF2e campaign and I'm probably going to be moving on to another system this year and likely won't ever use the books. There's another PF2e books I'm supposed to receive in March as well..
These experiences have led to me backing less system specific products and looking for more things that I can use for whatever I'm running. The other lesson for me to learn is to probably stop backing campaigns for physical items, but since I hate reading PDFs that's a lesson I've yet to learn.
I backed a few 5e campaigns for physical rewards and had stopped running 5e by the time they arrived because of delays. Recently I received the books I pledged for a PF2e campaign and I'm probably going to be moving on to another system this year and likely won't ever use the books. There's another PF2e books I'm supposed to receive in March as well..
These experiences have led to me backing less system specific products and looking for more things that I can use for whatever I'm running. The other lesson for me to learn is to probably stop backing campaigns for physical items, but since I hate reading PDFs that's a lesson I've yet to learn.

