This has happened to me a number of times too. I don't know why.
I don't know whether this is what's happening for you, Nielk, but here's a tip in case it helps.
A quote has an opening and closing "tag" that tells that it should be displayed in a separate box. Sometimes people accidentally delete, or type over, part of the tag that ends the quote. The poster may have accidentally deleted one of the brackets, for example, or put a space after the slash.
The tag to end a quote looks like this -
[/quote]
If you post a message and notice that the quote box doesn't stop where you want then you can go back and edit the message. Just insert
[/quote] where you want the quote to end. That should work (I hope I have not overlooked something).
By the way, it doesn't matter whether you use upper or lower case.