Drawbacks to over-pitching?

The last 2 brews I've done I've used dry yeast. In order to really get things going, I pitched 2 sachets both times (bee cave pale-notty; oatmeal stout-coopers).

Both resulted in very quick, violent fermentations that were complete in no more than two days (really closer to one). Is there any drawback to a super quick fermentation like this? It just seems that the extra buck or two is worth ensuring a nice quick start, but am not sure if there is such a thing as 'too fast'.

I've pitched on top of existing yeast cakes before and have seen beer finish in 24 hours. The main "drawback", if you want to call it that, is the heat generated by such a violent fermentation. As long as you can control your temps, there's no problem here. Just don't assume that since it's finished fermenting in 2 days that it's time to rack from primary on day 3. Leave it on the yeast for at least a week, preferably 2-3 weeks, so the yeast can clean up their "mess".

You can overpitch, but two dry sachets for 5 gallons is nowhere near the numbers you need to do such a thing.

I think I underpitched one batch and wish to never repeat that mistake. It came out estery. It's going away, but I'd definitely overpitch by a little than underpitch any day.

Overpitching can lead to a lot of esters as well. I think mostly because as Evan! said, it can get pretty hot in there with such a busy fermentation.

It can also create a battle amongst all the yeast in there for the sugars available. Instead of getting stressed because there is too much sugar, they get stressed because they are having to fight for sugar. Stressed yeast leads to ester production. Over-pitching isn't as bad as under-pitching, but can result in the same problems. Some say it can lead to earlier autolysis because the weaker cells are dying off very early, although I imagine this would still take a while.

The Blow Leprechaun said:

Overpitching can lead to a lot of esters as well. I think mostly because as Evan! said, it can get pretty hot in there with such a busy fermentation.

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Sure, and that's why I qualified my overpitching by saying "by a little." In cases like these, throwing two packs into a higher-gravity brew can't be overpitching by a lot, so I'd rather do that than take the chance of underpitching with just one pack.

I think pitching rates really depend on style. With a lot of belgian styles it is O.K. or even favorable to slightly underpitch to attain better ester profiles while with some lagers it is safer to error on the side of overpitching.

To the OP, I think your practice is just fine, keep doing what works.

Teacher said:

Sure, and that's why I qualified my overpitching by saying "by a little." In cases like these, throwing two packs into a higher-gravity brew can't be overpitching by a lot, so I'd rather do that than take the chance of underpitching with just one pack.

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Absolutely, I think you have to overpitch pretty severely to get a noticeable effect. I pitched a mock oktoberfest ale onto an entire yeast cake and got significant esters, even though the yeast was a really neutral one.

You can look up a pitching rate calculator to be exact, too - for example, you don't have to use the entire second pack of dry yeast.