BSON_ERROR_BUFFER_SIZE is defined as 64, but it's used in a few places to format bson_error_t message strings, which can be 504 bytes (including nil).

Set it to the proper length and use it in the bson_error_t declaration. There's a static check in bson-types.h to ensure the lengths add up correctly:

BSON_STATIC_ASSERT (sizeof (bson_error_t) == 512);

Review calls to bson_set_error and use the macro everywhere appropriate.