I was using the digg data set from the training course. I was following along with the training course, and got to the sample that says you can query by type.

Here's the console output:
./mongo
MongoDB shell version: 2.0.0-rc0-pre-
connecting to: test
> use digg
switched to db digg
> db.stories.find({"href":{$type});
...

Wed Aug 24 15:33:25 Socket say send() errno:0 The operation completed successful
ly. 127.0.0.1:27017
> db.stories.find({"href":{$type:2}});
Wed Aug 24 15:33:51 trying reconnect to 127.0.0.1
Wed Aug 24 15:33:52 reconnect 127.0.0.1 failed couldn't connect to server 127.0.
0.1
Error: error doing query: unknown
>

After I made the typo, note the missing 2nd closing brace after $type. I got the "..." from the shell. Then I hit ^C. Nothing happened, so I hit return. Then I got the following "operation successful." Then I tried the query again, without the typo, but the server was gone.

I repeated this to make sure, and the same thing happened again.