Serging inside corners doesn't have to be impossible. I learned this trick from the Palmer/Pletsch Serger Basics DVD. I'm illustrating it with a sample fly piece because that's usually one of the more difficult places to serge in one pass.
Pretend this is your fly piece (or just any other inside corner).
The trick to serging the inside corner in one pass is to pleat or fold at the inside corner, as shown in the pic below, so that the serging can be done in one smooth action. I'll show you how to do this on a serger in the next photos.
Here is the fly piece and I'm coming up on that inside corner.
I've pinched the fabric so that the inside corner is pulled into a straight line. It's kind of like making a dart, with the dart point ending at the inside corner.
I keep the fabric pinched as it passes through the blades and needles.
I'm past the inside corner at this point and am going to round the curve of the fly extension.
You can start from either end. I just happened to choose this end because it was a sample and the rest of the pants leg wasn't attached.
If you start at the other end (the top of the pants), you'll round the curve first and then make the pleat/dart at the inside corner.
This is how the inside corner looks immediately after being serged.
Unfold the pleat/dart and lay the fabric flat. If you're like me, you'll be smacking your forehead wondering why you didn't think of this before.