This is a sock knitting question that comes up fairly often, most recently in a Twitter exchange.
How to avoid the hole at the top of the gusset when picking up stitches?
(This applies only to a sock with a heel flap - the rules are different for a short-row heel. I'll cover short-row heel holes at another time, if you're interested...)
When you're at the top of the heel flap, have run out of edge stitches, and need one more stitch, many knitters have been tempted by the siren song of that lovely little bar that hangs out between the heel flap and the held stitches of the instep.
DON'T DO IT!
Picking up this one causes a hole.
Other knitters just stop at that point, and give up on the extra stitch. That doesn't help either, as it can leave a bit of a gap between the instep and heel flap.
Instead, keep going in the direction you were going...
That is, keep going up in the same 'column' you've been picking the stitches up in, in a straight line...
Yes, you're up into the leg of the sock, above the divide for instep and heel flap, but this is a good thing.
First of all, it keeps the gusset pickup in a tidy straight line.
And it handily eliminates both risk of hole and gap. No hole because you're picking up a fully formed stitch, and no gap because you're actually going to cover up the divide. And going that extra stitch makes for a better fitting sock, with a bit more fabric to fit comfortably around the heel.
No, it doesn't create any significant extra bulk.
As to coming back down the other side? Before I start picking up, I count up to figure out where I need to start. And I start up in the leg a bit, to match the first side.
And final tip - on the following round, I work the picked up-stitches through the back loop. It tightens and closes them up a bit, and makes the whole thing a bit neater.