The reason you don't remember an event or conversation from an evening of beer drinking.
Beerzheimer's phenomenon occurs when alcohol consumption inhibits the formation of long term memory. Beerzheimer's episodes can encompass the entirety of long time periods during which alcohol is being consumed, or can be fragmentary where only certain conversations or events are omitted from long term memory. Beerzheimer's episodes are often more pronounced during "sessions" of IPA or Barley Wine style ale consumption. Frequency of Beerzheimer's episodes is significantly increased when beer is consumed on an empty stomach.
"Hey, why didn't you call me back last night?"
"What are you talking about?"
"Yeah, I talked to you last night you said you had to go, but were going to call me right back."
"Sorry man, I had a few pints of IPA last night...never had any food. Got ugly. Beerzheimer's."
"Ah. You really need to build a base with some greasy food before you hit that stuff."