Please see mailing list discussion.
Main issue: not all catalog changes are detected, leading to caching issues. For example, security and adding/removing Dimensions (Time/Elevation).
I noticed that the GetCap document was not up-to-date. When I dug into it, I found that CapabilitiesCacheHeadersCallback was returning a 304 (i.e. no changes).
However, this is a timeseries dataset (ie. Time Dimension tab on Layer Publish).
(i.e. a GeoGIG or PostGIS dataset with a time column)
The issue is that the dataset is changing. A new Time value is inserted into the dataset - the getcap (set to LIST) should show this new value. However, the document is cached because the catalog hasn't detected any changes.
We talked about this earlier ("Time dimension on Vector Layer is SLOW") and decided that caching this information is inappropriate (and I agree) for this reason.