"here, we have the same mechanism again resorted to - and i think deplorably: for the mere repitition is distasteful, these drinks of grimhild are too powerful or too powerless: why not give one to atli too, and make him forget about the hoard!"
commentry on lines 17-28 of the lay of gudrun, in the legend of sigurd and gudrun.
jrr tolkien writing in the "if you could do that all along why didn't you just -" mode, about the over-powered forgetfulness potion that grimhild used to make sigurd forget about brynhilde (so that he'd marry gudrun), and to make gudrun forget about sigurd after he'd died (so that she'd marry atli [aka Atilla the Hun, who apparently existed contemporaneously with fafnir, loki, odin, &c]). thus he justifies removing the second use of the forget spell in his adaptation of the lay, replacing it with gudrun entering the marriage willingly but unhappily.
I love that he's complaining about this, the same issue that still riles the nerd community today. but as i've written before, tolkien's heroes get around having overpowered abilities by having powers that are poorly defined, and hardly ever used. he even uses a similar 'forgetting' spell cast by the dragon glaurung, that caused a similarly tragic marriage. is he being hypocritical, or is he just trying to write in the style of his heroes?
there are of course two responses to tolkien's rhetorical question: firstly, to counter it, it's a cursed hoard, and andvari's magic probably outranks grimhild's witchcraft. the second is *some other reason why, you can always just invent one in fantasy*.