Not sure this counts as a "common problem", but anyway...
Also, I read the rules
Tekkit 3.0.3 is trying to use the vanilla .minecraft folder as its folder (alternatively, WHERE ARE MY MODS???!!1)
Make sure you are using the latest technic launcher, that is, 1.0.1.3. For whatever reason, older launchers that will work fine for earlier versions and other modpacks do this.