If there is an item inside a relay that has no proper output, then any other item that gets put into the relay cannot travel either, even if there is a filter down the line that would allow it.

Example: You have a line of pneumatic tubes with a filter inline that will only accept smooth stone. If you put a stack of cobblestone inside the relay, the relay will take it inside it's internal inventory making it inaccessible to the player but not transfer it anywhere because the line sees that only smooth stone should be accepted. If you attempt to put Smooth Stone inside the relay while it's in this "stuck" state, the Smooth Stone will not travel either. You have to break the relay in order to get the cobblestone causing the conflict back.

Recommendation: Based on this behavior, the relay knows that it can't transfer anything. It shouldn't take the Cobblestone into it's internal inventory if it knows it can't go anywhere

Mod Version: 2.6.4
Forge Version: 14.23.5.2838
Other Mods in pack:

ModelLoader 1.1.6
Shadowfacts Forgelin 1.8.3