Originally Posted by MikeHNF
My guess (this is my guess and I don't know what I am talking about half the time) is you underpitched causing the off flavors. Half a vial for a 5 gallon batch is a bit short.
I agree. Even a moderate-gravity ale would require at least 160 billion cells for a 5 gallon batch. A perfectly-viable vial of yeast would have 100 billion cells, so it's safe to assume yours was somewhat less than that. If it was 80% viable, and you lost half of it, then in actuality you only pitched around 40 billion cells, or 25% of what you should have pitched.
My diagnosis is the yeast was severely stressed from being asked to multiply so dramatically, and produced off-flavours.
Next time, make a starter, or pitch 2 vials of yeast (without spilling