This error occurred trying to open some Java applications.
My Operation System is Linux Mint Nadia 64 bits (same base of Ubuntu 64 bits).
libXtst was already installed, but the error still happen. I could solve it installing the 32 bits version of the lib:
sudo apt-get install libxtst6:i386
Hope it helps you to!
Ajudou, Obrigado !
AEEEEEEEEEEEEEE MULECOTE VLW!!!!!!!