It wasn't a goof that Clara was standing after the wheelchair went down the mountainside. It was the shock of Heidi going over the edge that gave her the impetus to stand. It's all part of the storyline.
After the wheelchair rolls down the hill Heidi and her Grandfather run some distance down the hill after it before Heidi falls over the edge. After Heidi is rescued they turn round and Clara is standing behind them, at this point she is supposedly unable to walk.