I got caught beautifully by this today -- and it took a couple of hours to identify and sort the problem.
Here's what happened. These days I work with my page size set to 1024x768, so with that set as default page size in Tools > Option, I set my Chapters properties the same, and allow resizing without setting any maxima or minima. That works well -- normally, until I make a change which months down the track, I forget.
Occasionally I need to use a page which is slightly small than the default -- 1010x760, so I set up a new chapter to cater for this. Being lazy, I did not properly name this chapter so that the properties were obvious. First mistake!
Again, being lazy, I often duplicate chapters and pages so I have them on hand, all set with the required properties -- why? because it saves time
Today I grabbed a wrong empty chapter, added a page and after building it, previewed it. Of course, it previewed with the first page of my pub sitting behind it.
Thinking that I must have inadvertently or mistakenly set an action to do this, I spent an hour or so looking for the culprit. Couldn't find it
Then I remembered the chapter with the slightly smaller page size, and all became clear.
I post this as a reminder to myself and others about how easy it is to do something simple which later comes back and bites you