We've started seeing error during gclient sync on our bots:

[0:01:09] error: cannot lock ref 'refs/heads/playground/test': 'refs/heads/playground' exists; cannot create 'refs/heads/playground/test'
[0:01:09]  ! [new branch]      playground/test -> playground/test  (unable to update local ref)
[0:01:09]  = [up to date]      roll       -> roll
[0:01:09] error: some local refs could not be updated; try running
[0:01:09]  'git remote prune origin' to remove any old, conflicting branches
----------------------------------------
Traceback (most recent call last):
  File "/b/depot_tools/gclient.py", line 2146, in <module>
    sys.exit(main(sys.argv[1:]))
  File "/b/depot_tools/gclient.py", line 2132, in main
    return dispatcher.execute(OptionParser(), argv)
  File "/b/depot_tools/subcommand.py", line 252, in execute
    return command(parser, args[1:])
  File "/b/depot_tools/gclient.py", line 1894, in CMDsync
    ret = client.RunOnDeps('update', args)
  File "/b/depot_tools/gclient.py", line 1363, in RunOnDeps
    work_queue.flush(revision_overrides, command, args, options=self._options)
  File "/b/depot_tools/gclient_utils.py", line 1037, in run
    self.item.run(*self.args, **self.kwargs)
  File "/b/depot_tools/gclient.py", line 772, in run
    file_list)
  File "/b/depot_tools/gclient_scm.py", line 156, in RunCommand
    return getattr(self, command)(options, args, file_list)
  File "/b/depot_tools/gclient_scm.py", line 445, in update
    self._UpdateMirror(mirror, options)
  File "/b/depot_tools/gclient_scm.py", line 838, in _UpdateMirror
    lock_timeout=getattr(options, 'lock_timeout', 0))
  File "/b/depot_tools/git_cache.py", line 496, in populate
    self._fetch(tempdir or self.mirror_path, verbose, depth)
  File "/b/depot_tools/git_cache.py", line 471, in _fetch
    raise ClobberNeeded()  # Corrupted cache.
git_cache.ClobberNeeded


Examples: 
https://build.chromium.org/p/tryserver.webrtc/builders/mac_compile_rel/builds/588/steps/gclient%20runhooks/logs/stdio
https://build.chromium.org/p/tryserver.webrtc/builders/ios_arm64_rel/builds/15422/steps/gclient%20runhooks/logs/stdio

This effectively halts our whole continuous build. Doing a retry now to see if it was a transient error.