[obj] or {obj} usually indicates something the program can't correctly parse (means your text app has no idea what it is, or what to do with it).
OBJ inside a box means the program interpreted whatever it was as the Unicode object replacement character (U65532).
Either case means either the message was corrupted during transit, the file sent was in an image format that your phone/app can't handle, or there's a bug in your texting application. Any of these is possible, this stuff is complicated.
I wouldn't worry about it unless it happens all the time.