I was pulling into a parking lot. The attendant told me to back our 2017 Outback into a spot running along a fence and row of trees. I pulled forward, put my foot on the brake to fully stop. I then moved from "drive" to "reverse". Immediately, the engine started to rev very loudly as if my foot was on the accelerator (it was not!). The accelerator seemingly went to the floor and within a few seconds our car was in reverse at high speed. The only thing that stopped us was a maple tree and fence that we hit - about 15-20 yards behind us. The backend of the car is pushed in, including into the cabin, the rear window shattered etc. The fence is destroyed and tree damaged. My spouse, in the passenger seat, experienced a minor concussion and we both ended up with severe whiplash. If it weren't for the tree we would have just kept going. We are waiting on State Farm claim adjustor to see the car. I did inform Subaru (where the car was towed) what happened; they didn't seem particularly interested. We did get a police report. Thankfully, no one was hit. There were no signs of any problem with the accelerator prior to this incident. I am now, however, aware of numerous complaints and suits in related cases.