1.) "If it's gone down this much already, how much lower can it go?" (answer: Zero)
2.) "If it's gone this high already, how can it possibly go higher?" (some of the best companies grow for decades)
3.) "Eventually they always come back." (no they don't - there are lots of counterexamples)
4.) "It's only $3 a share, what can I lose?" ($3 for every share you buy)
5.) "It's always darkest before the dawn." (Its also always darkest before it goes absolutely pitch black. Don't buy a business just because price dropped and it is cheaper now)
6.) "When it rebounds to my cost, I'll sell." (The stock does not know you own it! Don't take it so personally Note: this comment is explained by the well documented psychological tendencies called loss aversion and anchoring bias which are talked about in Behavioral Finance. If you liked it at ten, you should love it at 6 so either buy more or sell)
7.) "What me worry? Conservative stocks don't fluctuate much." (There is no such thing as a conservative stock - the average stock fluctuates between 50% to 70% from its high to its low price every year. There is a graveyard where all the "conservative" stocks get buried. Companies and businesses change!)
8.) "Look at all the money I lost - I didn't buy it!" (Don't beat yourself up about the missed opportunities because it is not productive - when he managed the Magellan Fund, he almost never owned one of the 10 best performing stocks in a given year, but he did fine anyway).
9.) "I missed that one. I'll catch the next one." (Doesn't work that way)
10.) "The stock has gone up - so I must be right" or "The stock has done down - so I must be wrong." (Technical analysis is not worth much. So many people like something at 20 and hate it at 12 - never made much sense to him).
Peter Lynch has written (with co-author John Rothchild) three texts on investing, including One Up on Wall Street, Beating the Street, and Learn to Earn. The latter book was written for teenagers.