Mar 042013
 
select a_random_row
from   this_table
where  rownum = 1
and    i_wanted_the_first = 'Y'
order  by according_to_this;
This issue is slightly more insidious than last week's, though based on a similar misunderstanding. 
This query can appear to be workingly correctly, until the physical ordering of data changes 
(e.g. when migrating to a new environment); at which point it becomes apparent that the order by is done after rownum

For the correct way to do top-n analysis in Oracle, see this.

Feb 252013
 
select only_the_second_row,
       will_never_work
from   queries
where  rownum = 2;

no rows selected
A complete misunderstanding of ROWNUM seems to be endemic, judging from the number of forum questions asked about it.
The key thing to remember is it is set after processing the where clause, so you can't filter it there!

For a good explanation of this (including how to fix it) see this.