A rider came in today because "her gears suddenly started skipping."
"It made me swerve so suddenly I veered into traffic. I thought I was going to get hit," she said.
Something was clearly loose in the hub and cassette area. The freehub body had fractured at the lock ring threads, causing the cogs to detach from the splines. The whole mess was afloat. The chain could not possibly stay on one of them when nothing would stay in place.
The wheel was a first-generation Rolf from the mid 1990s. I believe those were Formula hubs. The freehub bodies were supposedly replaceable, although they could be difficult to detach from the hub shell.
I had never seen a freehub body fail in this way. If it happened while sprinting hard a crash would be almost inevitable. Fortunately, this rider was only hopping out of the saddle for a couple of quick pedal strokes to close a gap to her companion close ahead.