I have setup a hibernate project with a few classes using id generation by the @Id @Generated annotation.
The schema update fails due to the following error:

...
07.04.2006 15:12:07 org.hibernate.tool.hbm2ddl.DatabaseMetadata getTableMetadata
INFO: table not found: schema.hibernate_sequence
07.04.2006 15:12:07 org.hibernate.tool.hbm2ddl.DatabaseMetadata getTableMetadata
INFO: table not found: hibernate_sequence
07.04.2006 15:12:08 org.hibernate.tool.hbm2ddl.SchemaUpdate execute
SCHWERWIEGEND: Unsuccessful: create sequence schema.hibernate_sequence
07.04.2006 15:12:08 org.hibernate.tool.hbm2ddl.SchemaUpdate execute
SCHWERWIEGEND: ORA-00955: name is already used by an existing object
...

The error ocures because hibernate searches for existing sequences by the full qualified sequence name (i.e. "schema.hibernate_sequence") but it has retrieved the existing sequences from the database metadata with its unqualified names (i.e. "hibernate_sequence") . Hence it doess not find the existing sequence.
Then it tries to create the pretended non existing sequence and fails.
The relevant code ist found in org.hibernate.tool.hbm2ddl.DatabaseMetadata

public boolean isSequence(Object key) {
return key instanceof String && sequences.contains( ( (String) key ).toLowerCase() );
}

It should be somthing like this:

public boolean isSequence(Object key) {
if (key instanceof String) {
String keyString = (String) key;

if (sequences.contains( keyString.toLowerCase() ) {
return true;
}
String [] strings = StringHelper.split(".", keyString);
if(strings.length==3) {
return sequences.contains( strings[2].toLowerCase();
} else if (strings.length==2) {
return sequences.contains( strings[1].toLowerCase();
}
}