The problem happened when I accidentally typed "collection" between "use" and the name of the database I wanted to use. The relevant thing
is the blank space, of course, and not the word "collection".

The shell happily told me that it "switched to db collection docs". But, after I saved a document, I could no longer list the databases. I had to restart mongod to fix the problem.

Here's my mongo session showing that the problem persists after restarting the shell:
$ mongo
MongoDB shell version: 1.4.3
url: test
connecting to: test
type "help" for help
> show dbs

admin
local
> use collection docs

switched to db collection docs
> var doc1 =

{"a":1}

> db.coll.save(doc1)

bad char(s) in db name
> show dbs

assert: assert failed : listDatabases failed:

{ "errmsg" : "assertion: bad char(s) in db name", "ok" : 0 }

Thu Jun 10 23:17:39 JS Error: uncaught exception: assert failed :
listDatabases failed:

{ "errmsg" : "assertion: bad char(s) in db name", "ok" : 0 }

> bye

$ mongo
MongoDB shell version: 1.4.3
url: test
connecting to: test
type "help" for help
> show dbs

assert: assert failed : listDatabases failed:

{ "errmsg" : "assertion: bad char(s) in db name", "ok" : 0 }

Thu Jun 10 23:17:50 JS Error: uncaught exception: assert failed :
listDatabases failed:

{ "errmsg" : "assertion: bad char(s) in db name", "ok" : 0 }

And here's the console output:

$ mongod
mongod --help for help and startup options
Thu Jun 10 23:16:17 Mongo DB : starting : pid = 22524 port = 27017
dbpath = /data/db/ master = 0 slave = 0 64-bit
Thu Jun 10 23:16:17 db version v1.4.3, pdfile version 4.5
Thu Jun 10 23:16:17 git version:
47ffbdfd53f46edeb6ff54bbb734783db7abc8ca
Thu Jun 10 23:16:17 sys info: Darwin erh2.10gen.cc 9.6.0 Darwin Kernel
Version 9.6.0: Mon Nov 24 17:37:00 PST 2008; root:xnu-1228.9.59~1/
RELEASE_I386 i386 BOOST_LIB_VERSION=1_37
Thu Jun 10 23:16:17 waiting for connections on port 27017
Thu Jun 10 23:16:17 web admin interface listening on port 28017
Thu Jun 10 23:16:25 connection accepted from 127.0.0.1:58211 #1
Thu Jun 10 23:17:36 User Exception 10031:bad char(s) in db name
Thu Jun 10 23:17:36 Caught Assertion in insert , continuing
Thu Jun 10 23:17:36 insert collection docs.coll exception
userassert:bad char(s) in db name 0ms
Thu Jun 10 23:17:39 User Exception 10031:bad char(s) in db name
Thu Jun 10 23:17:46 end connection 127.0.0.1:58211
Thu Jun 10 23:17:48 connection accepted from 127.0.0.1:58227 #2
Thu Jun 10 23:17:50 User Exception 10031:bad char(s) in db name

And here's the data directory listing:
$ ls -l /data/db
total 8
-rwxr-xr-x 1 rstewart admin 6 Jun 10 23:16 mongod.lock*

After restarting mongod, the issue goes away.