Weird MySQL Behaviour

Last night I added a new log field to PotD and since I did it live on my prod instance, I wrapped it with a bunch of error handling so that if anything went wrong it wouldn't affect users, it just wouldn't log the new data.  (No, this is not my standard operating procedure – PotD is special in this sense).  Because of that silent failure behaviour, I wanted to monitor that the data was recording, so I ran this query:

select isPrioritized, count(*) from log_email;

Now, you can probably see exactly what's wrong with it, but it was late, and I didn't.  What's weirder, MySQL didn't throw an error, it just returned this recordset:

+---------------+----------+
| isPrioritized | count(*) |
+---------------+----------+
|          NULL |    15346 |
+---------------+----------+

So I spent a while trying to figure out what was wrong with my code, and it sure seemed correct.  Eventually gave up and went to bed, figuring that a few more hours without logging the data wouldn't hurt that much.  Upon getting up this morning I quickly realized what the issue was. The right query looks like this:

select isPrioritized, count(*) from log_email group by isPrioritized;

which returns this:

+---------------+----------+
| isPrioritized | count(*) |
+---------------+----------+
|          NULL |    15331 |
|             0 |        3 |
|             1 |       12 |
+---------------+----------+

I'm pretty sure the original query should error because of the aggregate function without a GROUP BY clause, but who knows.  Something to watch out for.