Cardiology took my HR up to 104% during a Bruce test in April 2009, and within 15 weeks I'd developed atrial fibrillation. As I didn't recognise it for what it was, and with my confidence buoyed up by a clear test, I increased my training intensity, so my AF got worse. When my health deteriorated enough to find myself back on the treadmill again in April 2012 I got another trip to 102%, and another clear test, but a few weeks later I found myself in an ambulance on my way to an AF diagnosis.
AF is quite good, I can reach 157% of MHR with no more effort than it takes to sit in the armchair.