Due to poor machining the thermistor hole was placed too close to the material feeding tube causing a breach which can allow for your printing material to leak.
A fair amount of information has been posted about this issue both on the Google+ Community and the RigidBot Kickstarter updates.
See here for more information and for images that will help you determine if the breach is your issue.
Users have posted some resolutions if you have received a rigidbot with a heaterblock that leaks
- Invent-A-Part has recognized the issue and is creating new heating blocks. If you have received a leaky heater block you can request a replacement from IAP, however, the lead time may be up to 6 weeks.
- Some users have found wrapping the thermistor with teflon tape will seal the hole enough to stop leaks
- Others have found putting a M3 screw in the opposite side of the hole stops the leaks
- If you have access to cnc mill you can make your own replacement part using the [Replacement Heating Block] file created by a RigidBot user
- Ensure that the nozzle is properly tightened and has not come loose.