The error messages thrown when state restore fails needs to be more explicit and clear of the actual reason.
At least 2 cases we've seen so far:
For example, currently, when restoring an operator state or memory-backed keyed state, the previous serializer must exist. If it doesn't exist, currently only a vague NPE is thrown, without a clear message of the actual reason.
If the restore failure was due to an incompatible version of a serializer's config snapshot, then it should report something more informative then: "Incompatible version: found 1, required 1."