Same with my INFP roommate from college. He'd often "yes" people to make them go away. We'd all be going out on a Friday night, and we'd practically beg him to come, and he'd always just say "ok, I'll catch up with you guys in 20 minutes" and then just never show up. Most of the time, INTPs would keep a deal like that, or just not make it in the first place.
I find this idea to be true with the INFP's and the INTP's I know.
The difference between Ti and Fi is also quite apparent. Fi folks are big into expression of their emotions, and often like to write poetry or music or something of the sort. INFPs are also always nice to people both to their face and behind their back. INTPs are gossip queens by any means, but they have no problem talking shit about someone if it's on their mind.
Most of my INTP friends enjoy writing poetry and playing instruments, and a lot of my INFP friends are gossips about people they don't like.