The only plugin that I was aware of to cause user problems was Oracle(TM) Java(TM). Basically, Chrome stopped using NPAPI, a good number of people freaked whenever their Java enterprise apps started dying. I'm assuming that Oracle has released a PPAPI plugin by now. Hopefully. The number of browsers supporting NPAPI just decreased by one.
"Extensions" and "Plugins" are a murky sort. Here's a quick blog post on the differences between them.