Oracle 10 column name resolution
It's realistically a bug in whatever version of 10g you're using that would appear to be fixed in whatever version of 11g you're seeing an error on both statements.  

I don't have a 10g database in front of me to test this.  I would wager, through, that you'd get an error in both cases if you used the old-style join syntax rather than the SQL 99 syntax.  That would imply that the problem is that Oracle wasn't translating from SQL 99 syntax to old-style syntax correctly and was missing the fact that the column was ambiguously defined.