Just some weeks ago we’ve learned that setting the hidden underscore parameter:
may cause trouble during upgrade. This parameter is used in very rare cases to have under all circumstances and situations this specified number of UNDO’s online. Now during upgrade this may result in massive latch contention – and there’s a patch available as well. Recommendation is to unset it during upgrade.
I don’t think that many people will hit this as I personally haven’t seen databases with this underscore in their init.ora or spfiles. So take this post more or less as a reminder for myself 🙂
Edit: I removed the mentioned bug number as it was incorrect (Dec 4, 2017)
Further Edit: As this seems to cause misunderstandings: This blog post is from 2012 and the issue described is first of all very rare, and second happened with upgrade to 22.214.171.124 only!!! (Feb 28, 2018)