We have a trigger tA (item iA), which depends on a trigger tB (item iB);
1. Everything is fine;
2. iA gets empty value and becomes Unsupported and tA correspondingly becomes Unknown, both with "Script returned nothing" error;
3. tB gets PROBLEM;
4. iA now gets correct values and becomes supported again, but its tA stays in Unknown state with the same error message;
5. tB returns to OK
6. only after iA gathers next value its tA will be refreshed and the Unknown state will be cleaned out.

The problem is that trigger tA may look like Unknown for long time (between 4-6) while it's actually not Unknown, which may mislead zabbix users.