It's obviously an error by Sa on whichever level you choose to look at it - coming for a ball he doesn't need to, making the wrong choice to catch, not assessing the conditions properly, not executing the catch when he makes that choice - but I can deal with that kind of mistake (as frustrated as I was at the time). Keepers all make mistakes across the season and invariably they end up in a goal at this level. You price it in.
It's getting rid of the absolutely bone-headed (passing the ball straight to opponents 20 yards out) or inexplicable (letting in shots that should be easily saved) errors that is the key with him. Goals that simply shouldn't happen under any circumstances and there were way, way too many of them. Touch wood, there haven't been any since Luton, where the root cause is crazy distribution from him to start with.