There is one case in which SO behaves strangely.
- A question is asked as non-wiki
- Less than 30 answers are posted
- The question is converted to wiki by the asker
- The question hits 30 answers
... then the answers posted before the wiki-ing of the question are never auto-converted to wiki.
If step 3 hadn't occurred, all answers would have been wikied.
If step 3 had been part of step 1 (i.e. the question had been originally asked as wiki), then all answers would have been wiki from the start.
But if step 3 occurs, then all answers posted before it occurs will never be wikified.
Here's an example: What is your longest-held programming assumption that turned out to be incorrect?
If you scroll down, you'll see that even though there are more than 150 answers, some of the ones on the front page are not wiki.