Issues during Oracle upgrade from 11.2.0.4 to 12c
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1. TEMP got filled , So its better to extend TEMP prior to upgrade 

2. During the upgrade perl file creates a its own readable pfile for multiple startups during the upgrade as you can see in screen shot (phase #44 & #47)

Upgrade was stopped as the process created pfile was not readable to open theDB  ,so DB has been started manually (startup) and conitnue  the upgrade process

Benefits of 12c Upgrade process :

1. Upgrade can be continued from where it has been stopped 

i.e., resume capability has been added to it

Suppose my Upgrade has failed at Phase#20 then i need to issue the command

$ORACLE_HOME/perl/bin/perl catctl.pl -p 20 -l ../../LOG_FILE_LOCATION catupgrd.sql

-p = For channel allocation 

-l = For Log location 

How resume works is 

it takes the information from logfile till the point of execution and then resumes from there

!!!....Hope this information helps....