i've always believed that you should introuduce people. if you're in a situation where you bump into one friend while you're with another, and they don't know each other, then it's just common courtesy to introuduce them. it's the least you can do so one person doesn't feel awkward if you're going to carry on a conversation with the other.

then one day i was out shopping with a friend, oh, let's just call her gloria. we're walking through a mall when i hear someone calling my name. i turn back, hey, that's my buddy's sister. let's just call her ida. after the initial hellos,

"oh, this is my friend gloria."
"gloria, this is..."

and i drew a complete blank.

"uhh....wow, i've completely forgotten your name! i'm such an asshole!"

after a few awkward laughs, we got the hell out of there. i knew her name when i first saw her, but it was gone 20 seconds later. i felt like such an idiot. but even so, it was pretty funny.