Technically saving register data, going to sleep, and then coming back and finding something underneath Windows has changed something is definitely an error, especially if it means the system's performance will suffer until it's fixed. It can also lead to not being able to wake a PC from sleep, so yes, it's an error, not a warning. Note that there could possibly be software drivers installed that can cause this, but it's rare. One thing I've found in my experience with this particular issue is that it generally happens on systems with older UEFI hardware, booted in "legacy" or BIOS compatibility mode. I don't know why this is, but I've yet to see it on a true UEFI installed/booted system. It could be that the latter is still somewhat more rare (mostly only new machines with Win8 preinstalled), but it's what I've noticed.
