Keeping a Timeline Straight

I'm impressed when a long-time series manages to keep a clear continuity. 

The X-men movies don't have good continuity. Which is distracting, but it doesn't stop me from loving many of the movies.  

Doctor Who does have good continuity. I haven't seen all of it. Indeed, many episodes seem to be lost forever, so no one can watch all of it. But I haven't noticed any glaring plot holes from the timeline (Doctor Who logic isn't always logical, but that's a different issue). In some bonus features, they say that people go back through old episodes, to make sure that everything lines up. 

There are also audio shows, books, and other stuff too. I wonder how those fit. 

I found the anniversary specials with The Doctor and Donna. It was great to see them together again. But they changed the word for "gravity." Does that mean that from now on, they can't use the word "gravity"" in Doctor Who? Or will they go fix it? Perhaps this has been answered already - I haven't caught up with the newer seasons yet. 

Star Wars wasn't created to be a series. George Lucas was happy just to get one movie out. He didn't even know if he'd be able to complete the first trilogy, let alone prequels. But since then, there have been more and more movies and series. Characters will pop up from older shows, which is always fun. (Even though I still don't buy the fact that Darth Maul survived being chopped in half). 

On the flip side, you almost have to watch all of the different series now just to understand some of them. For example, I watched Ahsoka when it came out. And only now that I'm in the middle of Star Wars Rebels do I realize that many of those characters were actually introduced before. I didn't know I'd need to watch Rebels for Ahsoka to make sense. So while I love that they build off of each other, there's just so much of it now that it's a real time suck if you want to actually follow one of the newer shows.