What are the effects of over-pitching? Where is the line for, say, a 1.050 beer?
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Yeast produce different flavors during the various stages of their lifecycle. Overpitching shortens or skips their "growth" phase (maybe a better name is "division" or "budding"). The bulk of a beer's esters are produced during this initial stage, so missing out on a fully-developed life cycle robs an ale of this often desirable quality.
A by-product of yeast metabolism is acetaldehyde, characterized by green apple flavor. During a full fermentation yeast will scrub much of this undesirable compound out of the beer. Overpitching could result in a noticeable amount of acetaldehyde left over.
To avoid the problem, use Jamil's pitching rate calculator and stay within 15-20% of the recommended rate.