Hah, I've encountered problems digging diagonally. There was this one time where there was no block underneath the one I had dug out and I fell right through a couple of levels. Luckily I didn't fall into lava!
So, now whenever I dig, I'm extra careful to try to fix things by adding blocks to patch things up before I continue to dig. The absence of blocks can really screw you up if you're not paying attention. I had to have my friend bail me out and build an elevator (we use a mod on the server that allows it).
All I ever do is log in to the server to play anymore. I hardly play single-player anymore. It's fun to keep adding to stuff and see the world that someone else is also working in grow. My Friend has completely remodeled his area, including the spawn and added a completely new building, which is still part of the boardwalk/bridge, and I think we're growing into an interesting community.