We developed an ASP.NET page (A) to be wrapped in an existing PHP page (B) using an inline wrapper. After a few days in which both pages displayed nicely, our page (A) suddenly got displayed twice inside the existing page (B). And even weirder was that the postbacks of the first occurrence didn’t work, and the postbacks of the second were executed outside of the wrapper.
We had changed some stuff the night before, so I finally checked the file line by line against an earlier version. The problem we eventually found was small and not too easy to find. But so easy to solve… Because of an external editor, the page’s (A) header had received an ID, probably because of the runat=”server” tag. Removing the ID tag was enough to have both pages working fine again.