Heights. I once took my daughters to the Ratho Climbing Centre for a go on the aerial obstacle course. It starts off with a zip slide and then you start walking across logs and the likes about 200 feet up. I managed the zip slide, got two or three steps on to the first log, looked down and that was that. I gingerly took a couple of steps back to solid ground and refused to go any further. Someone had to come and remove the harness from me and I made my way back to ground level. Meanwhile my daughters, age 11 and 8 completed the course and took great delight in my abject failure. Even now, 12 or 13 years later they take great pleasure in reminding me of this particular failure.