Using a password derived from your name, your department, affiliations, or other personal information. These can be easily guessed. Using words found in dictionaries. Even obscure words can be unsuitable for proper security. Using foreign words.
Most password cracking dictionaries are multi-lingual.
Using simple keyboard patterns like '12345678', 'Oooooo', or 'qwerty'. Not only are these easy to guess, but they're easy to determine just by watching you type it in. Repeating a simple word as a password. Passwords like 'FredFred' are checked for by automated password crackers. Using words, which may be contained in any of your files.