http://forum.kerbalspaceprogram.com/index.php?/topic/117090-sstulabs-low-part-count-solutions-orbiter-landers-lifters-dev-thread-11-30-15/&do=findComment&comment=2351884

It occurs when symmetry is on, and only the selected side meets this.
Reproducing steps:
(Maybe need Realfuels installed, not sure about it)

• Select a tank, change its size
• Attach it to a side decoupler(I've only tested with this), do not add anything in the bottom.
• Change its length, size, nose or mount for a few times
• Notice collided texture on one side

If it doesn't reproduce the issue, try detaching it and attach again. For me, it works.

I had it without real fuels or MFT.

Tracked it down to the method I was using to update symmetry -- was feeding it the symmetry counterparts the references for the data from the updated part, rather than their own data/objects.

Fixed in dev, and it all seems to be playing nicely now.