The idea of "fixed" is that it simulates something with roots or suckers that has attached itself to either the glass pane (if you think of Darwinbots as an aquarium) or ground (if you think of Darwinbots as an overhead shot).
A bot can be fixed, to my knowledge, in several ways:
1. It can write a non zero value to .fixed
2. It can develop alzheimers and have a non zero value written there
3. Another bot can write to .fixed through shots or ties
If you eliminate all these three possibilities, you might have found a bug.
Note that fixed bots pass on their fixed status to their children.