I hate when a sci-fi movie or series combines a different types of time travel/time continuum theories.
For example,:
1.they take the one crudely based on the theory of relativity - that every change or time travel was supposed to happen and it's actual the thing that caused the original event in the first place.
2.Then they take the theory where you can actually change the future or past with two variations of creating or not creating a time paradox
I don't have anything against these theories (although I prefer the first one), but even my beloved Star Trek Next Generation etc...Enterprise, DS9, Voyager or Star Gate lacked the consistency in whichever theory they chose (look at the end of 8th theory of SG1 for example). It shows how desperately was the whole time travel/change dependable on the actual story and often used to cover the plot holes.
Plus, I love Back to the Future trilogy, but the part where Marty's older siblings are slowly vanishing from his photo or the vanishment of doctor's tombstone from a photo as he/they change the past, is pretty much one of the dumbest things in sci-fi. Something similar happened in a Star Gate movie Continuum, where things and people were disappearing in a REAL TIME as the past was changing (past changing in a real time affecting the present? the scriptwriter should get a Nobel Prize or something).
Recently, I read a book by A.C.Clarke (yeah, The Space Odyssey guy) Rendezvous with Rama and it was written in a perfectionist style with the use actual physics even as changes in the plot. I recommend the book to all who find modern sci-fi lacking logic even in the already known things. And I also recommend it to those who think that you have to turn a blind eye, when something clearly stupid and illogical happens in a sci-fi.