Groovy Gotchas

These pass the sandbox compiler and then misbehave at runtime. They are the bulk of "it saved fine but does nothing" reports.

Handler names are strings

• subscribe(dev, "motion", "motionHandler"), runIn(300, "checkState"), schedule(cron, "poll") reference the handler by a bare string. A typo or a renamed-but-not-updated handler compiles clean and fails silently at dispatch time.
• When you rename a handler, grep every subscribe/runIn/runOnce/schedule string for the old name. The lint-review skill checks that each such string resolves to a defined method.

Groovy truth treats 0 as false

• if (offAfter) is false for both null and 0. Code that means "was a value provided?" but writes if (setting) mistakes a legitimate 0 for unset.
• Test presence explicitly: if (offAfter != null).

Null device inputs

• A non-required device input is null until selected. lights.on() throws when lights is null.
• Use safe-navigation (lights?.on()) or mark the input required: true.

Reserved and colliding names

• Don't name an input hubitatQueryString (reserved — holds the JSON of URL query params).
• Don't shadow built-in methods/objects (state, device, location, settings, log, app).
• name + namespace in definition must be globally unique on the hub.

Cross-instance data

• A @Field static variable is shared across all instances of that app/driver and is lost on reboot or code re-save. It is not per-device storage — use state for that (see rules/state-vs-attributes.md), or a ConcurrentHashMap keyed by device id if you genuinely need shared, rebuild-on-boot data.