OICA and Oracle choosing the wrong index

This is in reference to a new article by Jonathan Lewis about the OICA:

http://www.dbazine.com/jlewis18.shtml

It states that if you give Oracle a bad OICA it will choose the wrong index. 
Does anyone know why? I have run across this. I have cases where 95% of the 
time Oracle chooses the right index with a given OICA and for say 5% or less of 
queries Oracle chooses the wrong index. 

I've noticed Oracle picking bad join orders depending on how I set there. 
Especially with joins of 3 tables or more. 

Anyone have any comments or experiences? How much does CPU_COSTING eliminate 
this? We are still in development and have only delivered a small quantity of 
our application, so its not prudent to start messing with CPU_COSTING until I 
get a larger user sample. 

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