ok, not really sure what's up with this; I have a 12 speed that rides and shifts without incident, but recently I noticed that while I'm walking her the pedals turn the way that a fixed gear would. It doesn't seem to happen all the time, and I don't seem to have any issues while riding my bike. Any ideas?
Sounds like the ratchet inside the freewheel is getting gummed up. You can try flushing some oil into it by dripping it at the gap between the parts that stay still and those that rotate. But this will probably get worse so you should try to lube it.
If that fails, as someone (DaveM or cyclerUK?) pointed out in a different thread: you could try to service the freewheel, alas they are cheap (< 15 EUR mailordered) and it often is not worth the hassle.
This is potentially a dangerous fault, the chain can wrap up and foul the wheel whilst you are freewheeling and throw you off the bike, I would get it replaced as soon as, in the meantime, if you have to ride the bike, keep the pedals moving at all times.
Flushing out might work for a time, but the problem usually returns when you least expect it.