Put a vent hole in anything you weld that will be sealed up completely or air will heat up and expand and blow away your shielding gas or even blow out at the end of the weld bead.
Some machined joints that are sealed on one end will not even allow you to start welding because the fit is so good that the part is air tight before you even weld.
Other machined parts where a part is pressed in and bottomed out can give cracking problems because there is no where for the part to shrink. If you have to weld something that has been pressed in and bottomed out, make sure to add more filler metal than average to tacks and the final weld bead. That is to prevent the cracking that happens when you run a concave bead and the metal has nowhere to shrink.