In part two "Prom Night" (about 22 pages in), Margaret White is thinking back to a time when Carrie was not quite 1 year old. She almost killed Carrie, but her husband Ralph stopped her. Yet earlier in part one "Blood Sport" (about 8 pages in) we learned that Ralph died about 7 months before Carrie was born!
I too thought the same as you, that Stephen King had made an error, but I think that when he says that Ralph 'stopped' Margaret, he meant the thought of Ralph ie Ralph was dead so couldn't stop her physically, but mentally Margaret knew Carrie was all she had left of him and couldn't bring herself to do it?...
I too thought the same as you, that Stephen King had made an error, but after re-reading this part I think King means the thought of Ralph stopped Margaret killing Carrie, not Ralph himself.
We must also remember when reading Carrie, that her mother was a religious fanatic, as well as probably paranoid and most likely a schizophrenic, therefore when King says that Ralph "stopped" her from killing Carrie, in her mind he most likely did.....
This isn't a slip up related to this...but I didn't know where else to put it. If you have read Cujo and Needful things, a major slip up is the character of Aunt Evvie Chalmbers. Cujo is set first, and in it she dies...however, low and behold, a few years later, she appears in Needful things, alive and kicking. And SK makes it obvious that it is in fact the same character, not just same names.
These books have been writen for a long time to indicate that they are happening in different levels of reality. Castle Rock being to point of fluxuation in all of them...a tower with floors representing the different realms. Yes, you can read them independent of each other but if you read all of his books you will start to notice a theme. I would suggest starting with The Stand, and then reading the Dark Tower series. Then you could move on the The Tailisman, and then Black House.
And yes I have read them all many many times. To quote Anne Wilkes when she was talking to Paul Sheldon, "I'm your biggest fan"