After a basic, successful installation of Oracle 11g, I ran dbca again to enable Enterprise Manager on my database. The operation hung at around 67% for half an hour or more, so I clicked Stop to abort the operation. Things seemed to wrap up cleanly, EM was working, all was well with the world.

Then I started getting this dreaded error upon any attempt to connect in SQL*Plus:

ORA-12154:
TNS:could not resolve the connect identifier specified

I thought perhaps the database had been corrupted due to the earlier aborted operation, so I ran dbca again and deleted the database. Then I attempted to create a new database in its stead, using a clone of the template created the first time around. Unfortunately, the clone database operation fails at 50% with the exact same error:

ORA-12154:
TNS:could not resolve the connect identifier specified

How can I clean up the mess I have created, short of reinstalling Oracle entirely from scratch?