Vssadmin is dead, long live Diskshadow

Today while trying to compact a dynamically expanding virtual hard disk in Hyper-V, I ran into a cryptic error message that made bugger all sense:

Error Code: The requested operation could not be completed due to a file system limitation

I tried the same compact operation using diskpart instead, but got the same message, this time with the additional information that I could “See the System Event Log for more information.”

Imagine my surprise when I checked the System event log to find there was nothing about this.