Happens to me too, it's incredibly frustrating, particularly as I'm taking a degree in English Lit. so have to read a fair amount of books! I can get 'stuck' reading the same sentences or passages over and over again a ridiculous amount of times, although sometimes I find I'm not so afflicted and can get through a book or chapter fairly quickly - I'm not sure why exactly this happens only sometimes and not all the time, it's most bizarre.
I find that it can really detract from the overall experience of reading, when it turns it into a frustration rather than an enjoyable hobby. And that annoys me. On a related note, when I'm reading somewhere alone I usually almost whisper the words out loud rather than reading them in my head - like the re-reading of lines and passages, I just feel a compulsion to do it, otherwise I don't feel as if I've properly 'taken in' the words. I try to force myself not to do it sometimes, but I typically end up slipping back into to old habits... and it does get on my nerves.