Overwriting an existing save causes an error message. This happens for manual saves and quicksaves, unfortunately. For quicksaves, I have to go delete the oldest few to be able to use them.
This is on OS X 10.9.3 running the latest Steam version on a 3 year old Macbook Pro with 8GB memory and an SSD. I activated the Kickstarter key, if that makes any difference.
Let me know if there's anything you'd like me to do to help debug the issue.
