Sybase allows a foreign key between two tables to be of almost-the-same datatype. So table a's PK can be NUMERIC(10,0) and table b's column a_id can be INT.
While that may sound practical in certain circumstances, it's a potential performance killer when doing joins on those tables because of the implicit casting.
We just discovered that this was the cause of a major slowdown in a frequent query. Fixing it, the query runtime went from 1.7s to 13ms. Yay!
For comparison, Oracle requires exactly the same datatype on both sides of a FK relationship. At first I thought this was overly fascist, but now it seems like a good thing.
So since Sybase allows potentially stupid mistakes like this I'm inclined to implement a sanity test for this in our build tool so it doesn't happen again. That and code reviews, at least for code from external consultants.