Sorry if this is a known issue, but I didn't see it when searching.
I just completed my second playthrough and accidently caused two protagonists to appear at once!
Steps to reproduce (minor spoilers):
Examine the butterfly with the magnifying glass.
As soon as the butterfly flies away, click to the right to walk across the catwalk.
There will now be two protagonists on the screen for a brief moment:
![](http://chimbraca.com/machinarium/green_house_glitch.png)
Obviously not a show-stopper, but kind of fun none-the-less!
![Cheesy](http://amanita-design.net/forum/Smileys/default/cheesy.gif)