If you drop a databases that has engine set as Memory then the database re-appears after restart of the server.

• Create a database with engine Memory:
  CREATE DATABASE test ENGINE Memory

• Create a table in this database, again with engine Memory:
  CREATE TABLE test.test_table(a String) ENGINE Memory

• Drop the database:
  DROP DATABASE test

• You can check if the database exists:
  SHOW DATABASES LIKE 'test'
  Everything is fine, as the database is dropped.

• Restart the server:
  sudo service clickhouse-server restart

• Check again if the database exists:
  SHOW DATABASES LIKE 'test'

Query id: 59c2972f-90f1-477f-9610-1ca102044a22

┌─name─┐
│ test │
└──────┘

1 rows in set. Elapsed: 0.002 sec.

I checked on two recent releases and it's still happening.

Expected behavior
Dropped databases should not re-appear after restart of the server