https://developer.salesforce.com/trailhead/advanced_formulas/picklist_formulas
Has anyone managed to pass the challenge?
Gives error:
Challenge Not yet complete... here's what's wrong: 
The validation rule does not appear to be working correctly. Marking IsEscalated to true and Priority to Medium did not fire the validation rule.
Testing that same case myself (and some permuations) the Validation rule fires correctly I think...
Here is my validation formula
AND(
ISPICKVAL( Status , "Escalated"),
OR(
IsClosed,
IsClosedOnCreate,
NOT(ISPICKVAL( Priority , "High"))
)
)