Most Helpful Guys
Yes, I believe so.
The most common version I have personally seen is a setup where one person is in a relationship with someone that does not meet all of their needs, so they supplement with a third party.
That supplement is usually emotional. But often is physical (in the case where their partner has sexual dysfunction, they get that need taken care of by a third party).
Some friendzone situations are actually love triangles where the third party is used for emotional support. The sexual needs are being met by the primary partner, so the third party is limited to emotional intimacy.
Most Helpful Girls
I don't know, I've never witnessed one in a real life setting. I guess it's very common for someone to have a crush on someone who is already involved with someone else, but I find it difficult to believe that people would be so rude that they would act on it as well.