I thought that too. In previous circuits that use optos i once connected one the wrong way and it never worked after i corrected it. I dont know for sure that the reverse current caused it as i had never turned it on before wriring it the wrong way. It could easily have been damaged by overheating during soldering or it might have been a bad component when it arrived.
The triacs/optos are inside the light holders and its very fiddly to replace them. I have thousands of diodes laying around and they cost almost nothing so its well worth including them just incase.
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