A common error that a lot of diy electricians make is to assume that
all reds are lives,all blacks are neutrals and all greens are earths.
Also that all reds go together,all blacks go together and all greens
go together.

I think the real problem is that many competent electricians and diy
enthusiasts fail to sleeve the cables correctly to indicate to others the
possibility of them being live. A bit of red tape round a core goes along
way particularly in lighting circuits involving multiple switching.