OK, you're working away on your model and everything is going along fine. Then you notice that, after you drew a line from the corner of one plane to another corner on a different plane, it is in the wrong place - inside of your model where you can't get at it! Or you may have joined to pieces of your model with a line and ended up with part of the line inside one of the pieces. You can erase the line outside of the geometry, but not the remainder inside.
These lines will not show up in your final model when you publish it, but they can be REALLY annoying as you continue modeling. If you want to get rid of them, here's a trick that I use all the time:
1 - Pick a surface close to where the line is, preferably a good-sized one, and change the color to the transparent gray in the Glass section in the drop-down list of the paint tool. It's already selected by default, since it is the first one in the list.
2 - Once the surface is transparent, you can zoom or move right through it! Keep zooming and panning until you can see that annoying line as a white line again and then delete it.
3 -Zoom back out and change the surface back to the color that you want it to be.
4 - Done
Sometimes when you're trying to zoom through the transparent surface, it will stop you before you get where you want to go. Hit the Esc button several times and try zooming in again. That usually works.