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.)