2 Redstone Repeaters next to each other on delay 4 causes the receiving repeater to stay powered

(Note: The loop means the repeater staying powered until the power is cut off)

2 Redstone repeaters next to each other causes the receiving repeater to loop if there is a source of power that alternates between on and off very quickly.

If the receiving repeater's delay was changed to anything else, it would break the loop. If changed back to 4, it will loop again.

If the sending repeater's delay is changed to 1, the loop is broken. But if the delay is changed to 2,3, or 4, the stay powered loop will happen again. And if changed from 1, the sending repeater may become stuck as powered too.