I had my first clipless moment last week. I'd just climbed a big hill (Wyche Cutting for anyone local/interested) and decided to stop at a bench for a breather and to say hello to another cyclist who was sat there. I managed to cross the road, hop up the curb and was about to ride up the grassy embankment to the park bench at the top. Unfortunately, the grass was wet and I didn't hit it straight on, so the bike slid down and left whilst I continued on. I landed on my side on the (soft) grass, picked myself up. No damage done, but the could see the other cyclist laughing inwardly at me.
Oh, and HeartfeltDawn, although I love my SPD's (and have only had the one incident in over a year of daily commuting use) I'd say you'd probably be better off without clipless pedals. Some report they lose confidence doing 'tricky' manoveurs with clippess that they'd do fine in flats - and if you're not 100% confident that clipless and your medical history is right for you, then they're probably not right for you.
If a single fall, or twist of the knee whilst unclipping might cause you months of rehab - no amount of pedalling efficiency is going to make it worth it.
EDIT: Might be worth looking into those small toe clips that only cover part of the foot - might give some benefit without restricting "get-out-ability" (Although I have no experience of them to seek advice from others
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