Validation Rules

Account

Billing / Shipping Zip Code if in the US

Rule Name :: Billing_Zip_Code_US / Shipping_Zip_Code_US
Description :: US Zip code must be in 99999 or 99999-9999 format.
Error Message :: US ZIP code must be in 99999 or 99999-9999 format.
Error Location :: Field [Billing Zip/Postal Code] / [Shipping Zip/Postal Code]

AND(
OR(BillingCountry = "USA", BillingCountry = "US"),
NOT(REGEX(BillingPostalCode, "\\d{5}(-\\d{4})?"))
)

AND(
OR(ShippingCountry = "USA", ShippingCountry = "US"),
NOT(REGEX(ShippingPostalCode, "\\d{5}(-\\d{4})?"))
)

- NOTE: You could add either of the below in the OR segment. - ISBLANK(BillingCountry) - ISBLANK(ShippingCountry)

Opportunity

Closed Lost Reason

This one is fairly simple, but it assumes you have a 'Closed Lost' custom field (please add a brief Description to your custom fields). - Rule Name :: Closed_Lost_Reason - Description :: I used the same one as the 'Closed Lost' field - Error Message :: Closed Lost Reason is required. - Error Location :: Field [Closed Lost Reason]

AND(
 ISCHANGED(StageName),
 ISPICKVAL(StageName, 'Closed Lost'),
 ISBLANK(Closed_Lost_Reason__c) 
)