Sequence:

0) Oracle 10.2.0.2 EE
1) user scott hit his tablespace quota on tablespace users; he remains connected to the instance
2) DBA tries to add some space to the schema: alter user scott quota 2000m on users;
3) DBA session with the alter user statement will hang until scott session is not closed (or killed)

There are several variations of this scenario: alter tablespace add datafile, allocation of undo segments in undo tablespace etc.
Fixed in 10.2.0.3 / 11g R1.

Regards,
Ales