mix ANSI and Oracle JOINs?

Hi List, 

somewhere in the back of my brain I have the ruleset 'do not mix ANSI and 
Oracle JOINs within the same SQL'. 
As I have no reference for this I'd like to crosscheck such memories from time 
to time as a) this memory is quite unstable and b) as Oracle changes, it should 
be checked from time to time.
I searched MOS and asked google, but did not find a real reason.
Should I wipe this memory? 


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