I discovered this while testing out Spark for the first time. I wanted to see how the new features held up without Panels, out of curiosity - so I disabled all the Panels modules that were turned on, then went to the homepage. Only to find:
Fatal error: Class 'panels_display' not found in /Users/erica/repos/spark/profiles/spark/modules/spark_demo/spark_demo.module on line 205
I shouldn't have been able to do that so easily. Turns out the Spark Demo module only lists every_field as a dependency, which populates most needed dependencies down the chain of modules Every Field requires, but Panels/Panels IPE don't get mentioned by any of them. Panels should be added as a dependency in spark_demo.info so that Panels cannot be disabled while Spark Demo is enabled.
Minor problem, really, but we want Spark to look good to new users and be easily testable. I will add a patch in a bit.