Synthetic chemistry is one of those disciplines in which there is no substitute for experience. Frequently, something arises for which there is no accepted procedure, so a high degree of resourcefulness is required. No one is particularly good at it when they start (usually as an undergrad), so a few fuckups are more or less expected. Some people make more mistakes than others, and some mistakes are more serious than others. Derek Lowe has a fantastic collection of these stories, not all about undergrads, and I highly recommend that you read them.
I've been a bit on the cautious side in the lab, and I usually think about what I'm doing, so I don't have too many stories of my own to share. But here is something that happened to me very, very recently. Remember my craptacular oil bath? Well, those Sonogashiras were in tiny little flasks, and the original level of oil in my bath wasn't quite high enough to effectively heat them. So I decided to add a little more. These were oil baths I'd inherited from the last guy who was in my hood, so I'd never had to fill or refill them. I knew we used PEG (poly(ethylene glycol)) for this, so I grabbed a bottle off the shelf and dumped some in, then turned up the heat a little and let it melt into the oily mess that was already there. The cute little Sonogashiras worked beautifully, but I had something else to set up that was in a 1L flask, so I poured some of the oil back into an empty bath and stuck the flask in there.
The PEG had solidified. What I didn't know was that the PEG we usually use for oil baths was not quite the same as the white crystalline-looking PEG that I had added to mine. When a labmate pointed out the greasy-looking carbowax PEG, I knew what I had done. The molecular weight on the correct PEG was around 600, but the crystalline stuff was around 8000. Quite a difference.
 Often, this requires fixing things around the lab when they break. I am always looking for an excuse to break out the screwdrivers.
 I think being cautious drives my boss a little nuts. He's very much a cowboy chemist.