I have noticed occasions of when the .dtBase2 icon will randomly depixelate or completely change to that of another application. Included is screen capture for reference.
That picture gets the award for today’s weirdest anomaly. 
It cannot be a problem of DEVONthink itself, as the assignment of icons is a chore of Finder/OS X, which seems to have become completely bewildered.
And how!
Boy, sometimes I long for the days when, if you encountered this, all you had to do was rebuild the Desktop.
Maybe repairing the disk or permissions using Disk Utility? Maybe it’s time to run Disk Warrior just to do some spring cleaning? Or perhaps a simple logout/login will clear this uup.
-Tod
The equivalent mantra for OS X is: clear the Launch Services database (a.k.a. “the Registry” in other lands).