Warren Buffett's first stock investment

Warren Buffett's early investing lesson: Was selling too soon a mistake or a useful experience?

Buffett's first stock investment taught him something about the difficulty of deciding when to sell. After he sold his shares, the price continued to rise.

This creates an interesting debate: a trade or investment should not necessarily be judged only by what the price does after we exit. The quality of the decision also matters.

Discussion: How do you distinguish a sensible exit from an emotional one? Should a trader evaluate an exit based on the original plan, or on how much additional profit the market later offered?

I'd be interested to hear how others approach this problem.