ClassNotFoundException during ImageJ startup

Continuing the discussion from Trouble launching Fiji in MacOS:

The OS X distribution of Fiji has never included a JRE. It was previously the case that OS X shipped its own version of Java 6 (“Apple Java”), which is what Fiji used. But newer versions of OS X no longer include Apple Java—you have to download and install Java 7 or Java 8 from Oracle.

For more details, see the OS X section of the FAQ.

There was a bug in the ImageJ Launcher which resulted in such an error (which, BTW, is not the reason Fiji failed to start up here), but it was fixed some months ago. Is this user running the most recent version of Fiji?

That looks like a missing ome-xml.jar, one of the Bio-Formats supporting libraries.

And that implies a missing scifio.jar, or dependency thereof.

Maybe the user enabled an update site which shipped conflicting versions of some libraries?

Note that imagej/imagej#97 documents this very issue—ImageJ startup being too fragile. The plan is to improve the robustness of ImageJ startup.