I have also blacked out well within my limits and once had a bad samba from 16m - but if I am honest on both occasions I was very tired and run down from working a long stint so I guess you really need to be honest about what is pushing yourself on a particular day - it may be close to your PB on a day that you are feeling healthy and strong but on a day when you are tired, or ill, or hungry or its winter and you haven't been keeping that fit - it might be a lot less. Now I would say to everyone to never train alone. And a swimming pool lifeguard or anyone else does not count as a buddy unless he is within an arms reach of you and entirely understands what you are doing.