Not had it yet but did have a brain fade moment in cloud once. My attention was diverted by something for probably only four or five seconds and when I looked back at the AI I was in a 30 degree left turn. Oops I thought, no probs so I applied left aileron...then when the AI went the wrong way the message hit home and I recovered.
Time was probably about two seconds, it seemed like two minutes and I can still clearly remember the 'WTF is happening' feeling. It isn't nice.
Lesson learned: don't get diverted by anything, fly the bloody aircraft.