The bug

Placing down an unconditional chain command block then running it with a button makes the output below it "Unknown command. Try /help for a list of commands", although the impulse block activating it also has no command in it and no output field is shown.

To reproduce

Place two command blocks down, an impulse facing into a chain command block.
After verifying that there is no output and command in both blocks, activate the impulse command block. The impulse will not show any output, but the chain will show "[Time] Unknown command. Try /help for a list of commands".