Disinterested vs uninterested
disinterested = impartial, no personal stake; uninterested = not interested, bored
Economic vs economical
economic = relating to the economy; economical = not wasteful, cost-effective
Alternate vs alternative
alternate = every other one (adj) or take turns (verb); alternative = another option
Principal vs principle
principal (adj/noun) = main, or head of school; principle (noun) = fundamental rule or belief
Examples
A disinterested judge is fair. An uninterested judge is dangerous for different reasons.
The economic crisis was severe. The solution was economical and efficient.