I had a stupid run out the other day while cross country schooling and I was absolutely furious for letting it happen. Though there were no flags on the jump, when analysing why it happened, I realised that I had let my horse fall through his shoulder in the approach (its very subtle but its enough at a skinny). I realised that when I was jumping him normally he would often fall through his shoulder to the left and that I probably got away with it while show jumping but at this skinny I ended up with a stupid run out.
I am now going to work really hard to stop this bad habit and I am going to be much more concious about is the horse jumping straight all the time because at a skinny you could be caught out by your bad habits so my tip of the day is to work out which way your horse drifts through their shoulders and then to work hard to adjust it.
You only need to canter a pole a few times on the floor to see if you approach, jump and land straight to be able to work out which way you drift even if it is only slightly.