I think I had pushed new code to secure001 and then wandered off to do something else.

This left requests hitting one of the hosts randomly, then loading panels from other hosts randomly. Likely, at least one of those requests would hit secure001, and at least one request would hit a different secure host. These two requests returned different copies of core.pkg.css, and when Chrome got the second copy it had to recompute page layout.

This is something we can monitor in the long run, but ultimately operational error.