  Since May 2006, the two STELLA robotic telescopes at the Izana observatory in
Tenerife, Spain, delivered an almost uninterrupted stream of scientific data.
To achieve such a high level of autonomous operation, the replacement of all
troubleshooting skills of a regular observer in software was required. Care
must be taken on error handling issues and on robustness of the algorithms
used. In the current paper, we summarize the approaches we followed in the
STELLA observatory.
