There are two special cases in tolerance messages:
```
# There are two special values for tolerances:
#  * 0 - The tolerance is unspecified and will remain at whatever the default is
#  * -1 - The tolerance is "erased".  If there was a default, the joint will be
#         allowed to move without restriction.
```
http://docs.ros.org/api/control_msgs/html/msg/JointTolerance.html