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1. Dxx can't jump to a row with a higher number than the row it's on. There seems to be a check that limits the row paramter to the length of the current pattern, and since Dxx changes that length, it clamps down higher values that might be valid in the pattern being jumped to. Maybe the limit should be the global pattern length, not the current one? (Or at least this behaviour should be documented.)
2. If using Bxx and Dxx only Bxx takes effect. This can be inconvenient if you want to loop to the middle of a pattern. Obviously you can get around this by splitting that pattern into two patterns, but it might be nice not to have to do that. (Or if not, maybe note this in the document.)
I was going to post this but you beat me to it. :p
I'm in favour of fixing the behaviour rather than documenting the erroneous behaviour. Most other trackers I've had a go at let you use Bxx and Dxx (or their equivalent) on the same row, and they all let you skip to any row with Dxx.
Those bugs have proven slightly irritating in trying to loop imported NSFs properly. :\