Thanks for all the input! FWIW, this is the video I used as a guide. He suggests doing all 16 on the drive-side and then moving over to the other.
Off by one spoke makes sense. I'll take a look at this later today and report back.

FWIW, the wheel is a Alexrims R390 which indeed has centerline spoke holes. I guess that made it much easier to make this sort of mistake.