I think any of us that teach aerobatics can tend to get too complacent - you know the thing - send your student out to preflight the plane without checking it thoroughly yourself. This is, if anything, a cautionary tale about checking the plane out immaculately yourself, and explaining to the student why you do so.
We know we can fly the manoeuvres and we assume that by putting the right inputs in that we will be safe when we fly. I personally will now become more cautious than ever about double checking for stress/ wear etc when a student preflights for me.