When a dispenser is hooked up to a repeating circuit, but NOT FIRING at the time, and it's full of 9 stacks of fire charges, the following happens:

"Phantom fire charges" appear for a split-second in midair.

When actually in front of the dispensers, the following happens, in addition to the "phantoms":

The game's framerate drops from around 75-85 to just 10-20, and sometimes even lower.
The tick rate slows. Redstone currents no longer work immediately. A button press will open a door eventually, but it takes up to 20 seconds for it to do so, even when there is NO delay. The same with pressure plates and redstone blocks when applied to anything.
I sometimes glitch back to the last known bug location. For instance, a door failing to open (as described above) could send me back to that spot even 10 seconds after I've left the area.

For now, I'm choosing to keep the dispensers loaded at only 5/9 capacity. The glitch does not occur if I do not fully load them.

EDIT: The tick slowing has been fixed to some degree, but the framerate still fluctuates to very low levels. The dispensers can be fully loaded, I think, but I still won't try it.