Unpopular names and dirty mustaches

יומא פג: 

ר"מ ור' יהודה ור' יוסי הוו קא אזלי באורחא ר' מאיר הוה דייק בשמא ר' יהודה ור' יוסי לא הוו דייקו בשמא כי מטו לההוא דוכתא בעו אושפיזא יהבו להו אמרו לו מה שמך אמר להו כידור אמר ש"מ אדם רשע הוא שנאמר (דברים לב) כי דור תהפוכות המה ר' יהודה ור' יוסי אשלימו ליה כיסייהו ר"מ לא אשלים ליה כיסיה אזל אותביה בי קיבריה דאבוה אתחזי ליה בחלמיה תא שקיל כיסא דמנח ארישא דההוא גברא למחר אמר להו הכי אתחזי לי בחלמאי אמרי ליה חלמא דבי שמשי לית בהו ממשא אזל ר"מ ונטריה כולי יומא ואייתיה למחר אמרו לו הב לן כיסן אמר להו לא היו דברים מעולם אמר להו ר"מ אמאי לא דייקיתו בשמא אמרו ליה אמאי לא אמרת לן מר אמר להו אימר דאמרי אנא חששא אחזוקי מי אמרי משכוהו ועיילוהו לחנותא חזו טלפחי אשפמיה אזלו ויהבו סימנא לדביתהו ושקלוהו לכיסייהו ואייתו אזל איהו וקטליה לאיתתיה היינו <דתנן> [דתניא] מים ראשונים האכילו בשר חזיר מים אחרונים הרגו את הנפש ולבסוף הוו דייקי בשמא כי מטו לההוא ביתא דשמיה בלה לא עיילו לגביה אמרי שמע מינה רשע הוא דכתיב (יחזקאל כג) ואמר לבלה נאפים 

Yoma 83b (loose translation, my own)

Ribbi Meir, Ribbi Yehuda, and Ribbi Yosse used to travel together. R' Meir had a habit of judging people based on their names, and R' Yehuda and R' Yosse did not. One day, they stopped in some city and checked into a local inn.
"What's your name?" They asked the concierge.
"Kidor," he told them.
R' Meir didn't trust him, because of the pasuk in Devarim that says, "For a generation (Ki Dor) of uprising they are, children who you can't trust."
R' Yehuda and R' Yosse entrusted him with their cash over shabbat, and R' Meir did not. Instead, he went and buried his wallet in the graveyard. He happened to bury it, however, right by Kidor's father's grave. That night, Kidor dreamt that his father was telling him to come take the cash buried near his head. Spooked out by the dream, he asked his rabbi guests what to make of it. R' Meir quickly assured him that dreams are meaningless and that there is no cash in the graveyard. As soon as he convinced Kidor to stay home, he ran to the cemetery to look after his hidden wallet so he can dig it up as soon as shabbat was over.
The next day, as they were checking out, the rabbis asked Kidor for the money they deposited with him.
"Money?" said Kidor. "What money? I know not of this money about which you speak!"
"What did you expect from a guy named Kidor?" R' Meir teased them.
"Why didn't you warn us?!" they asked R' Meir.
"Well, when it comes to names, nothing is for sure. I didn't want to spread rumors about this guy over a hunch."
As they were checking out, they noticed that some of the lentils which Kidor had for breakfast were still stuck in his mustache. They were suddenly inspired with a plan. After they left the motel, they stopped by Kidor's home and had a talk with his wife.
"Hey, you know that cash that your husband brought home last night?" They told her. "We're here to pick it up!"
"Okay," she responded, "but how do I know that my husband really sent you?"
"He told us to tell you that this morning you had lentils for breakfast instead of what normal people eat for breakfast."
On the grounds of this sign, she gave them the cash. When Kidor came home and saw what his wife had done, he beat her to death. About this incident the berayta states, "(Not washing up before a meal caused someone to eat pig, and) not washing up after the meal caused someone's death." (Others tell the story that he divorced her, Hullin 106a.)
From this moment on, all three of them were quick to judge people based on their names.

The moral of the story is always wash your hands and mustache after you eat, especially if you're planning on ripping someone off that day.
(On a slightly more serious note, Rabbenu Avraham b. haRambam says in his monograph on aggada that the moral of the story is that our rabbis were so smart that they could outfox all the jerks that tried to rip them off.)