I once had Grant with about 2 divisions blocked in Columbus KY in '61 two turns in a row by a stack of between 3 & 5 cavalry. Right when they were trying to move toward Henry the cavalry sprang in and stopped then in their tracks. Being that they were early war cavalry I decided to ignore them and just move on. The next turn I plotted Henry again with the cavalry starting in my region and of course the same thing happened, but this time the cavalry was on its way back to Humboldt, from where they had come.
I've never been able to find anything in the rules describing blocking movement (also happens if you try to cross a river against enough opposition) or in the forum and I though it had to do with the cavalry, maybe comparing cavalry forces. But if a supply unit can block movement, maybe the game is just counting horses

. No, seriously

.
But it's a real bug if an empty corp can block movement, or maybe the game thinks that long-faced Hood looks like a horse

; or maybe it's just Grant

. We all know how much he loved horses. Maybe every time some new ones come into view he just has to stop and admire them

.
But seriously, if anybody can say anything about how blocking works it would be much appreciated.