The following quote is taken from Geoff Eddy's description of his 'Geoff Mode' remix of the JSW game engine.
Am I right in my interpretation that it isn't possible to have Invalid Arrows in Geoff Mode (because of the aspect highlighted in bold)?
Arrows work differently
In Matthew Mode, the time of firing of any given "arrow" guardian is specified within the guardian class, and the arrow reappears every 256 ticks. In Geoff Mode, the time of firing is specified within the guardian's start byte in the room data, and reappears every 128 ticks. This means that, in practice, you only need one arrow class for each direction (right and left). You also lose the ability to specify the arrow's row at pixel level, but I really don't think anyone's going to miss that.