I know there was a recent thread about the right/wrong ways to tighten compression fittings to prevent water leaks. I don't know if this was mentioned but I'll throw in something that caught me out again yesterday: when you take a compression fitting apart, it is easy to accidentally loosen the one at the other end of a piece of pipe, just enough that it's hard to see the leak but you can find a small puddle under the bathroom sink a while later. So it's worth checking all of them in the vicinity.
(In this case, I undid the fitting at the bottom end of a flexible connector so I could slip the hollow hexagonal tool over the connector & tighten the nut on the wobbly tap. I discovered later that the compression fitting on the top of the service valve had worked very slightly loose.)