Panic sells reveal themselves with hindsight, but the last memorable panic sell - that feeling of fear - was last October when the oil stocks were diving hard. In hindsight I think it was a lot of forced selling from over-leveraged investors, but I exited several positions that would've been better to hold for rebound, and I've learned that ideally I'd be buying (have cash available) when I feel that way. If I'm doing a trade on discretion, I've learned to try to poll my emotions for whether I'm feeling fear or greed.
Like others have said, I've also sold a stock only to check back on it months later and it's gone down much further - even bankruptcy. Financial stocks are particularly dangerous for me in this regard. Surprises can seem to come out of the blue and demolish them.