Explanation
An item version can only be made immutable if all of its child {0,keyword,products} are immutable. While the API was attempting to check in an item version, and working from the leaves of the tree upwards, it found an item version with a mutable child {0,keyword,product}. This might occur if another user or session performed a check out of that child {0,keyword,product} while the check in operation was in progress.