Wasn't in my essay, but in my attempt to very briefly describe a research experience, I put "PI" in without thinking. My advisor, with whom I cosigned everything during the project (the grant proposal, the IRB proposal, etc.), was okay with it and basically said "it won't be an issue; don't worry about it," but I expect that will get me a lot more flack than your little misspelling will get you!
Honestly, though, everyone is bound to make mistakes in their app no matter how often they check and recheck it. Heck, the AMCAS software creates errors -- I had it keep cutting off my punctuation in the experiences even though it was several characters shorter than the limit. I finally just let it go with endings such as ) instead of ). Honestly, unless some adcom is ridiculously meticulous, they are HIGHLY unlikely to even notice the difference.
For your incite vs insight, well...funny, but I wouldn't worry about it!