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I made a cursory search for a list of bugs present in Oracle Application Express 4.0.1 or 4.0.2 and fixed in 4.1, but I could not locate this little bug.
Here it is: say you need to copy page 1 to page 3.
Apex will duly search and replace the occurrence of the various forms of page items named like "P1something" so you'll end up with page 3 and all your nicely renamed page items like "P3something".
I just found out that at least in the item condition "Expression 1" attribute, if you have an item like "P100something" (which refers to page 100, not page 1), it will be changed to "P300something", which is wrong. I didn't perform other variations on this scheme, but it looks like the replacements are made simply checking for a "P" followed by the page number being copied, no matter if there are other digits after it.
I couldn't verify if this occurs also in Apex 4.1, but chances are it will, I guess this find and replace stuff must be there since ages.
PS: on the same subject, I also noticed that any page items like &P1something. located inside the attribute "Post element text" won't be renamed at all. And this is another little quirk.