I recently picked up some out-of-date WLP800 dirt cheap and decided to make my first lager. Since the yeast was really old and mrmalty said the viability was 10% (or less) I nursed the yeast back to health by first making a 500mL starter. It took about 4 days to finally get going, but it worked. I then stepped up to 2L, fermented out and cold crashed for 24 hours. The following day I brewed a 5 gal batch of 1.057 wort. I then realized that a 2L starter was probably not going to be sufficient for a lager of this gravity. A quick check at mrmalty showed I needed 3.8L of starter. I also decided to bank a little bit of yeast for later use.
So here's what I did: I decanted the beer of the starter yeast and added 1.75qt cool, sterile water and resuspended the yeast. 1qt went into a sterilized ball jar, cooled and pitched into the freshly made wort. 1/2 pint went into the yeast bank and 1 pint went into a fresh 2L starter. I let that starter go for 18 hours, cooled and pitched about 24 hours after the initial pitch.
I know this is not the best practice and I don't plan on doing it again, but I'm curious about *how* bad this really is. I see now that I underpitched by about .8L of starter or 21% so that's probably gonna cause some diacytel. What about the first 24 hours of severe underpitching? Will this cause big off-flavor problems, or does the yeast get going so slowly that the additional infusion 24 hours later will smooth out most of that?