Q: I'm getting a ton of "Unable to find the DECORATE class '[some actor]' to inherit from, while parsing '[some other actor]:[DoomEdNum]'" warnings. What's wrong?
A: This happens when a DECORATE actor is inherited from another actor, which is not defined in game configuration and is not defined in any included resources. To fix this, add "(g)zdoom.pk3" as a map resource. Don't forget to check "Exclude from testing parameters" chekbox while adding it, otherwise (G)ZDoom won't load your map. "(g)zdoom.pk3" can be found in (G)ZDoom folder.
Q: Dynamic lights defined in GLDEFS are not rendered in Visual mode / I'm getting a ton of "Light declaration not found for light '[light name]' ('[source]', line [n])" warnings. What's wrong?
A: GZDoom Builder doesn't include GZDoom's GLDEFS lights definitions. To get rid of these errors, add "lights.pk3" as a map resource. Don't forget to check "Exclude from testing parameters" chekbox while adding it, otherwise GZDoom won't load your map. "lights.pk3" can be found in GZDoom folder.
Q: I don't like the "fake contrast" (zdoom.org/wiki/Fake_contrast) GZDB uses to render sector walls in Visual mode. How can I disable this?
A: GZDB's Visual mode emulates (G)ZDoom's default rendering preferences (in (G)ZDoom, Options -> Display options -> Use fake contrast is "on" by default). You can change this behaviour by adding "evenlighting" or "smoothlighting" property to the "map" or "defaultmap" definition in the MAPINFO. More info about MAPINFO properties supported by GZDB can be found here.