Why do my stitches wander off line halfway down a long run?

It's usally both things at once, and the longer the run the more they compound. Your wrist rotates the iron a little every punch, and your whole arm drifts as you shift your body down the gusset. On a card wallet it never gets long enough to show. On a full gusset it shows by halfway. Cheap irons also tend to have slightly bent or loose tines, so one corner of the tool bites harder and nudges the row sideways without you feeling it. What fixed it for me: draw a proper guideline with a silver pen or scratch it with a wing divider, and don't trust your eye after that. Rest the iron's edge against the line on every single punch, not just the first few. Then every four or five punches, stop, stand up, and sight down the row against the line like you're sighting a rifle. You'll spot the creep while it's still one hole wide, and one hole is easy to correct by angling the next punch slightly. Once it's three holes out you can't fix it without it looking patched. Also make yourself stand and walk around the bench rather than sliding your arms along the leather. The lean happens when you stay planted.

One more thing that helps on gussets specifically: punch the straight runs from both ends toward the middle. Errors in the middle of a gusset are way more forgiving than errors right at the corner where everthing gets seen togather. On the basketweave, always plan it before you cut, not after. Measure the panel on the pattern, figure out how many stamps fit at your spacing, and shift the start point so the leftover space splits evenly at both edges instead of running out on one side. scribing a border line with a groover first gives the weave a hard stop and hides a slightly tight last row. And yes, stamp it flat before assembly. Trying to basketweave around a sewn corner is how I learned this lesson the slower way.

One thing that helped me with drift was taping a little depth stop on the iron so every hit goes the same distance in. When the punches get shallower partway down a run the tines start riding on top of the previous holes and that pushes the row off too. Also check the iron itself, lay it flat on the bench and look down the tines against a straightedge. I found one of mine was actualy bent from a bad mallet day and no amount of careful holding was fixing that. For the gusset I clamped the whole panel down to a marble slab so the leather could not wander while I worked along it, made a bigger difference than anything I did with my hands.

The thing that finally sorted my long runs was a cheap stitching pony. Before that I was clamping the gusset flat to the bench edge and my punching hand was doing all the alignment work, which is exactly how you end up leaning without knowing it. Clamped vertically I could keep my forearm square to the leather the whole way down and the drift basially vanished. Mine was like thirty bucks, nothing fancy, just enough to hold the work still. Might be worth trying before you blame your hands entirely.

Another option if the iron keeps fighting you: set your dividers to the stitch spacing and scratch every hole position on the guideline first, then punch to the marks instead of chasing the row. Slower, but on my last messenger gusset it was somethig like 70 punches a side and the two rows ended up dead even. Also try wrapping the handle with a bit of leather or bike tube, a slick handle rolls in your grip way easier than you'd think.

The trick that saved my long gussets was overlapping the tines into the last two or three holes from the previous placement before punching the next set, which locks the iron into the existing row and makes slow rotation basically impossible to accumulate. If the tines drop in clean you know you're still on line, and if they resist you catch the creep immediately instead of three punches later. I also scribe a small reference mark on the top of my iron's handle facing me, so any rotation of the tool is visible at a glance before I swing the mallet rather than after. Between those two habits my drift went from embarrassing to essentially zero.