Broken colibri skin on a subwiki when the rest of the farm is in flamingo

On a farm, I have the following situation:

• all the wikis are using, directly or as the parent skin of a custom skin the flamingo skin
• one wiki has nothing at all in the skin field in preferences - the flamingo skin is used on this wiki
• one particular wiki is using colibri skin

The settings for the skin are as follows, in xwiki.cfg:

xwiki.defaultskin=flamingo
xwiki.defaultbaseskin=

From time to time we get, on the wiki with colibri skin, an issue that resembles this screen: http://jira.xwiki.org/secure/attachment/29036/IssueOnUpgrade.png (attachment from XWIKI-11187), with "glyphicon macros" and all.
We cannot manage to reproduce the issue in this configuration, but it did happen.
This farm was recently upgarded to 6.2.4 but after the upgrade it was fine and the server was also restarted since.