Originally posted by: Brahmaputra
Karna's curses are narrated in detail in Shalya-Karna conversation in Karna parva & Narada- Pandava conversation in Shanti Parva. Still, despite being cursed by Parashurama, Karna remembered Brahmastra three times correctly & used against Arjuna in battle but failed to remember once - Karna remembered it & used twice, then he forgot, then he again remembered it and used. So Parashurama's curse did not work at all. Had it worked, Karna would never have remembered it at all while fighting with Arjuna.
I agree Jamy Parashuram's curse to Karna does seem to be an interpolation since karna is able to recall Brahamastra in is final duel with Arjuna but there are few other possibilities as per me
1)The curse is an interpolation.But yet it is mentioned in different parvas so there is consistency in these mentions.So if it is an interpolation whoever has added it has done a good job of keeping it consistent
2)The curse is actually true but Karna wasn't cursed regarding brahamastra but Brahamshira.That would explain why Karna was able to recall Brahamastra and would make karna the only person other than Ashwathama and Arjuna who had Brahamshira
3)The curse is actually true and somehow Karna was powerful enough to overcome it .It would explain the following line in the duel when karna recalls the Brahamastra
Trembling at the shock, Karna then displayed his activity to the utmost of his power. Steadying himself by a powerful effort he invoked the brahmastra
It could also be that karna does something that requires a lot of power to recall Brahamastra hence overcoming the curse