I'm guessing the bit walked due to the differences in the hardness of the woods?
Yes, they did walk due to the greater hardness of the plugs. My bad, as I usually work a little smarter than this. Two things, at least, that I should have done differently:
1) Drilled the holes out larger and used a dowel diameter that placed the correctly located hole, wholly within the plug wood, and
2) Started small and work my way out in 2-3 increments so the "path of least resistance", which a drill will follow, was more controlling than the difference in the wood density/resistance.
I do 2) frequently, but was lazy I guess, even though I have the dowels that would have made 1) easy. I think I just didn't want to drill a large enough hole to start with.
Steve...thanks...however, I will need to defer to Rick regarding how the drilling did not go as planned. There are too many variables for me to offer any conjecture. As you might already know, drilling into end grain takes on its own life.
Yes, drilling into end grain presents its own challenges. I used a centering/squaring drilling fixture, where there is a platform for holding the drill square and legs that can be extended down the board and when each are held tight against the side of the board, centers the drill.
To widen the base against which the squaring platform rests, to provide a broader surface to guide the squaring, I placed boards of the same thicknesses on either side of the stick I was drilling (see photos above). This had worked well in the past and did so when I drilled the previous ten end holes. I think I got careless and didn't hold one of the legs firmly against the guide board, probably the one underneath where I couldn't see it.
I think this effort of plugging and redrilling was doomed to failure due to my using a plug that wasn't wide enough so the new hole was entirely encompassed by the dowel.
I've used this fixture for going on 40 years without this much trouble. I have another, more modern one, that can tilt and has recoil springs. I don't like it and don't use it. This old one works well, but I did have to remake the shaft after it got bent... somehow...