As I started mentioning in comment, += actually modifies the list in-place and then tries to assign the result to the first position in the tuple. From the data model documentation:
These methods are called to implement the augmented arithmetic assignments (+=, -=, =, /=, //=, %=, *=, <<=, >>=, &=, ^=, |=). These methods should attempt to do the operation in-place (modifying self) and return the result (which could be, but does not have to be, self).
+= is therefore equivalent to:
t[0].extend(['world']);
t[0] = t[0];
So modifying the list in-place is not problem (1. step), since lists are mutable, but assigning the result back to the tuple is not valid (2. step), and that's where the error is thrown.
