Thesis proposal near thesis defence
As MadJack says in the comments, this is really something that you and your advisor need to sort out with the department.  Part of an advisor's responsibility to their students is to make sure that they don't end up in easily avoidable bureaucratic screw-ups like this.  So yes, you've failed, but your advisor has failed as well.

First thing to do, then, is to sit down with your advisor and figure out how serious a problem this is.  If it's something like JeffE suggests, and due to recent regulation changes, or if it's a requirement that is more pro-forma than serious, then there are likely good ways to deal with it.  For example, you might end up following the letter of the law and in fact having the proposal exam be essentially a dress rehearsal for the defense (and that might be considered to be OK).  Or the proposal exam might end up finding some problems that really do need to be addressed before you defend, and you will be grateful---you might be delayed, though, and that might or might not be a problem (you don't say where you are: in the US, delay is generally not a problem if your advisor can fund you; in some other countries it can be a major issue).

If the department is not flexible, however, and wants to make an issue of your failure to comply with the letter of the law, then the resolution will be very particular to the department, and that is something that you and your advisor will need to navigate together.