Ah, the very thread!  I have a cactus, or rather a clutch of them off one root.  It started as a little phallic finger (thanks to Mother for that present!) but I repotted it after it fell down the back of the fridge for 9 months and nearly died.  It went mad, producing lots of new phallic fingers in brighter green, but the original one went brown.

My question is: can I prune the supposedly dead fingers away?