Of the three stories I have heard like this, two of them ended up involving rohypnol. If you aren't paying attention, it looks like a girl who didn't know her limits. If you are paying attention, it looks like a girl who went from tipsy to sloppy blackout drunk in the space of a Black Russian.
The ultimate tell is a crippling hangover. Just be aware - if you ever find yourself far drunker than you feel you should be, there may be an agent responsible other than alcohol.