Latest version of geometry.sty breaks beamer

So if you updated all your packages in one go and finds that your beamer presentations don't compile anymore, this is why. Don't panic though (42), there's a workaround: put the following line before \documentclass{beamer}:

\makeatletter\let\ifGm@compatii\relax\makeatother
\documentclass{beamer}

Read this post for more information.