Since “but” and “however” perform the same function in a sentence, it’s not appropriate to use them together. Suppose you have written “but the cake he made for my birthday, however, was his old girlfriend’s favorite flavor, not mine.” Revise this to use just one or the other. For instance: “but the cake he made for my birthday was . . .” or “the cake he made made for my birthday, however, was. . . . ”