I haven't played for a while (or on this computer at all), so I don't know how long this has been going on. But if I press mouse button 4 while I'm holding the left mouse button (mouse button 1), it acts as if I'm holding down mouse button 5. Since I'd tried to map dash to mouse 5 and shield bash to mouse 4, this resulted in me continuously dashing as soon as the cooldown finished and not seeing a shield bash unless I'm not holding mouse 1. I tried mapping action and defend to mouse 5 and confirmed that doing the above is acting as if I'm holding mouse 5. The game thinking that mouse 5 is held stops once I actually press mouse 5. I tested this in other programs and have not encountered this behavior, so it seems specific to Spiral Knights or Java.

Edit: Upon further testing, this is happening when the right (mouse 2) or middle (mouse 3) buttons are held before pressing mouse 4 as well, and NOT if mouse 4 is the button being held first.