Two bugs here, kind of in the same vein. The first bug we have found was not quite replicated by us, but it has happened twice now, with the second time being captured on video.
Summary: The Quarterstaff Q ability Concussive Blow does not consume all the stacks when knocked off a horse entity, causing each successive Q hit to act as a third stack stun -- but never consumes the stacks.
Actual Result: When reaching the third stack off Concussive Blow, I was stunned and the 2 stacks from concussive blow never consumed to Buy Albion Online Gold; they eventually fall over time, but each successive Concussive Blow strike stunned me during that duration (but did not refresh the timer either).
Expected Result: After reaching a third stack from Concussive Blow, I'd have expected the 2 stacks I had already to be consumed and the following Concussive Blows to start stacking anew.
Steps to Replicate: Unable to replicate but this has happened at least twice now. We tried to replicate it a few times, but we are unsure as to what factors it would require. All we know is that it appears to happen in regards to riding a horse and being knocked off it while Concussive Blow is stacking. (See video below: when I am removed from my horse and stunned, the debuff "slot" for concussive blow stacks moves from the last to the first, but remains 2 stacks)
Video: Youtube Video here
Quick Sum Up of what we think happens here:
It appears to be an issue with horse entities/objects and transferring debuffs to the parent object (or child object? depending on hierarchy) when the horse is removed. It has only ever happened when on a mount and being removed, but again we cannot fully replicate this. Other factors could include de-sync from server and all that.
If you also notice, at one point I would have been able to blink/cast/move whatever, I try to spam my instant heal on myself or blink to a location. However, this is prevented due to.... bug number two:
Summary: Being stunned while moving and trying to cast sometimes causes desync from the client/server, preventing casting or using of abilities until you move (resyncing with the server).
Actual Result: When moving and trying to cast heals on myself, when I get stunned, when the stun duration is over I can no longer cast spells; it tries but auto cancels the spell/ability until I move again.
Expected Result: When moving and casting, after I get stunned and the duration is over, I'd expected to continue to be able to cast spells/use abilities without having to move.
Steps to Replicate: Move around and self cast abilities (or on others if you can swing that setting), while someone who can stun (we tested with quarterstaves and hammers) stuns you. Without moving after being stunned, try to cast again with the help of Cheap Albion Online Gold. You shouldn't be able to until you actually move to re-sync your client with the server.