It is possible for one to have ongoing issues with that hell if it only happened here and there not sure it would even qualify as a legit issue with short term memory...I am also quite forgetful and could easily see myself saying I'll do something and totally spacing it out, or I say I'll help someone with something or perhaps I borrow a little money so I have to pay back and I legitimately forget...or if I am sharing a cigarette with someone and forget to pass it to them....but don't mind being reminded and I feel bad when I forget something or end up unable to follow through.

Best thing to do though is maybe bring it up with this person, let them know you are getting bothered by them saying the'll do this or that and not following through but be civil and be willing to hear their side if it is memory problems you're probably just going to have to accept that and keep it in mind rather then assuming they didn't because they don't care or something....also sometimes people can end up getting too much going on like thinking they can do this and that, but have sort of over-estimated their abilities or forgot other responsibilities that are more urgent that then have to be taken care of first.

Also though some people are flaky, they make one plan....then make a different plan, hard to get ahold of them and even organize something ahead of time....my sister is like that, but I don't think its really something she can help or intends. Either way don't think this would relate to a Personality Disorder, it would either have to do with short term memory problems which can have different causes....or they are being lazy, or just blowing you off.