The message that always causes a chill over spine when doing things in production is ORA-12154: TNS:could not resolve the connect identifier specified.
Today I happened to see it after installing 11g client on top of the existing 10g. PL/SQL developer stopped to work not finding oci.dll, copying oracle dll files to the bin folder was enough to move to the next step: ORA-12154.
It turns out that when you have multiple oracle clients installed you have to resolve the tnsnames.ora location by having a TNS_ADMIN environment variable pointing to the folder where tnsnames.ora resides. The most helpful link on the subject I found is http://forums.oracle.com/forums/thread.jspa?messageID=3742352.
After this fix everything worked as expected. Note that if you are installing xcopy of 11g client TNS_ADMIN is a must anyway (http://plainoldstan.blogspot.com/2008/03/xcopy-installation-option-for-net.html).