Last year a car failed to realise that:
A - the road ended at a junction
B - Said junction was a roundabout
C - cars were on the roundabout and had right of way
It was pretty scary but thankfully being fully aware of the risk I was 90% expecting it (10% refusing to believe anyone could be so stupid and oblivious). I think I missed him by about 50cm. One of those times that your reaction is to brake, and fast, rather than to honk your horn. I suspect he carried on driving off without even realising how close he was to causing a serious accident. I suspect a lot of accidents on roundabouts happen not between said retarded drivers and the "victim", but rather the "victim" and driver behind. I was lucky no one ploughed into me too!
Yesterday when I drove in to uni I nearly went into a lady on a roundabout as she had forgotten her car had indicators. I know it was my fault really, you're taught to not rely on people using said indicators, but given she was driving pretty fast and road position suggested she'd be going straight on, it was quite a surprise when her car started to swing round the roundabout