Didn't actually undo the bolt all the way. Doing the job tomorrow and just planning ahead to see if it was tight enough to justify removing that coolant hose that's in the way of the bottom e10 bolt to get a socket in there. In which case I'd need to pick up some more coolant. Just a full turn to see how easy they break loose, which was pretty easily, then tightened it back up. So couldn't have cross threaded it. Just an alarming sound that makes the nightmare of a snapped bolt head come to mind. I'm assuming it's a reusable bolt as it's steel and has no blue on it.