rake test:units で次のようなエラーが出てしばらく悩んだ。
/usr/local/lib/ruby/gems/1.8/gems/sqlite3-ruby-1.1.0/lib/sqlite3/errors.rb:94:in `check': SQL logic error or missing database (SQLite3::SQLException)
from /usr/local/lib/ruby/gems/1.8/gems/sqlite3-ruby-1.1.0/lib/sqlite3/resultset.rb:76:in `check'
from /usr/local/lib/ruby/gems/1.8/gems/sqlite3-ruby-1.1.0/lib/sqlite3/resultset.rb:68:in `commence'
from /usr/local/lib/ruby/gems/1.8/gems/sqlite3-ruby-1.1.0/lib/sqlite3/resultset.rb:61:in `initialize'
from /usr/local/lib/ruby/gems/1.8/gems/sqlite3-ruby-1.1.0/lib/sqlite3/statement.rb:158:in `execute'
from /usr/local/lib/ruby/gems/1.8/gems/sqlite3-ruby-1.1.0/lib/sqlite3/database.rb:211:in `execute'
from /usr/local/lib/ruby/gems/1.8/gems/sqlite3-ruby-1.1.0/lib/sqlite3/database.rb:186:in `prepare'
from /usr/local/lib/ruby/gems/1.8/gems/sqlite3-ruby-1.1.0/lib/sqlite3/database.rb:210:in `execute'
from /usr/local/lib/ruby/gems/1.8/gems/sqlite3-ruby-1.1.0/lib/sqlite3/database.rb:620:in `rollback'
... 13 levels...
from /usr/local/lib/ruby/1.8/test/unit/autorunner.rb:200:in `run'
from /usr/local/lib/ruby/1.8/test/unit/autorunner.rb:13:in `run'
from /usr/local/lib/ruby/1.8/test/unit.rb:285
from /usr/local/lib/ruby/gems/1.8/gems/rake-0.7.1/lib/rake/rake_test_loader.rb:5
落ちついてコードをじっくり見ると、fixtures の引数に渡している名前を typo していただけだった。