Worst SQL sins you've ever seen, go!
I'll start.
db had six tables, most of the queries were fairly typical. But one section of the website forced a monster of a query that smashed all six tables together, selected all of it, and did all the filtering in the where clause. Something like:
select * from a,b,c,d,e,f where a.foo = b.bar and.....
Those of you who grok SQL are now rightfully terrified.
It took over 45 minutes to execute and caused the server to swap aggressively. We noticed it after it set off alarms for load, swap usage, and memory a few times.
I rewrote it for the customer using left joins which dropped the processing time to a few seconds at most. The client loved it, the developer insisted my query was missing important data.