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This depends on two things. Firstly, do we mean HEAD as in the section of the document that precedes BODY in HTML, or to we mean headings, as in <h1>This is a Level I Heading!</h1>
In the first case, the browser will end up being thrown into quirks mode. Most modern browsers won't allow you to simply ignore the validity of a document. Once in quirks mode, the browser will take a "best guess" at what you meant. Considering that </head> is immediately followed by <body> there's s better than fair chance that the page will render with very little repercussions. Still, because of the way quirks mode is browser specific, there's no way to be sure how the rendering will occur from one browser to the next.
If you drop the </h1> slash, the browser will also go into quirks mode, because the W3C specifications don't allow for nested H1 tags. Again, quirks mode in unpredictable, but I would suspect that the rest of the page would be rendered as though it were inside an H1 tag, because, really, it is.
