Olivier Messiaen
Messiaen was in a lot of pain near the end of his life.  He needed operations on his back.  He still managed to finish a piece called  Éclairs sur l'au delà, which was first performed by the New York Philharmonic Orchestra six months after the composer's death. Messiaen had also been composing a concerto for four musicians he felt particularly grateful to: his wife Loriod, the cellist Mstislav Rostropovich, the oboist Heinz Holliger and the flautist Catherine Cantin. This was nearly finished when Messiaen died, and Yvonne Loriod orchestrated the last movement with advice from George Benjamin.