This warning is correct but pointless for a hot bucket. If Splunk incorrectly attempts to parse hot buckets for the time endpoints in some cases, then numbers which indicate the oldest and newest times of the bucket are not available to be parsed. In other words, for hot buckets, attempting to parse the names will produce this warning.
My best information (grain of salt here) suggests that we are not currently aware of the reasons why Splunk would parse hot buckets, and we have not been able to produce the problem via any means in Splunk 6.1.
It might be advisable to manually investigate these buckets and see if there's anything unusual about them (missing, truncated files, etc.) that would lead to a problem needing further investigation by Splunk. However it could be that this is just a messaging problem was fixed by changes not specifically targetted at this message between early 5.0.x and 6.1.