User blog comment:Kirkburn/Technical Update: February 17, 2016/@comment-4530178-20160218085539/@comment-126761-20160218135347

As a quick bit of background on that fix: when we converted everyone to the new avatar system towards the end of last year, a small percentage of accounts unknowingly stayed on the old system. When those old avatar URLs started expiring recently, the issue was noticed - so we ran a script to identify the leftovers and get them converted earlier this week.