When I was playing around with localization within Velocity Macro I received some bad exception when I was trying to get a localized string whose key doesn’t exist.
“Can’t find resource for bundle java.util.PropertyResourceBundle, key”
It was even worse that the rest of my template wasn’t rendered just because of a missing localisation key. That’s pretty bad and I would rather see one non-localized key instead of nothing at all. When those localisation keys are generated dynamically these exception can come even more often if you change some value in one spot for example not thinking about that this one is used for localisation.