There have been a number of reports of “Assertion Failed” error messages when shooting Nikon and Camera cameras tethered to Capture One since the release of OSX 10.6.8.
Until this is isolated to specific models, or resolved by a new OS or Capture One release we recommend dSLR shooters not update to OSX 10.6.8.
Those who follow our advice of Backing Up Before a Shoot by creating a fully bootable (and thereby restorable) backup of your computer before any major updates can roll back to their last update.
Update: Capture One Pro 6.2.2 has been released and should fix most or all of these situations. Provided you have time to properly test afterwards it should now be safe to update C1 and OSX without interruption to tethering.