One of my users received a couple of these:

Local Failures were detected.  No action is required.
This e-mail was generated for technical support purposes.

Sync Type: Delta Sync

Sync Token (before request): 1863

Sync Token (most current): 2633

Store: Zimbra - <user>

Computer Name: LASPC408

Version: 5.0.2767.10

Error IDs:
id (69) type(tag)
The attachement shows (this is only the top of it):

<SyncResponse md="1231454257" token="2633">
<tag d="1231453845000" ms="1864" color="0" md="1231453845" u="0" name="Experiment notification" id="69" rev="1864"/>
<m ms="1865" md="1231453855" l="2" id="803" rev="1865"/>
<m ms="1866" md="1231453856" l="2" id="804" rev="1866"/>
<m ms="1868" md="1231453860" l="2" id="806" rev="1868"/>
<m ms="1870" md="1231453865" l="2" id="807" rev="1869"/>
<m ms="1870" md="1231453865" l="2" id="808" rev="1870"/>
So it looks like something borked on the tagged message. Any ideas what to do to fix this?