AssetService declares this field:
static final DateFormat CACHED_FORMAT = new SimpleDateFormat("EEE, d MMM yyyy HH:mm:ss Z", Locale.ENGLISH);
However, DateFormat objects are explicitly NOT thread safe. We've encountered this in our in-house concurrency testing. We'll have to patch our local 4.1.1 to fix this before deploying our application.
a quick search did not find any other references to SimpleDateFormat that look like they could have the same issue.
A simple CommonsPool object pool of date format objects would fix the problem.