The last town George and Lennie worked in was Weed, which is a little town in Siskiyou County, several hundred miles north of San Francisco. When they first appear in the novella they are on their way to a ranch in the Salinas Valley, which is over a hundred miles south of San Francisco. So they have traveled around four hundred miles together without seeking work. This is evidently because of the trouble they had in Weed, a subject George refers to in both Chapter One and Chapter Three.
George brings up the subject of their trouble in Weed when they are camped by the river in the first chapter.
Hie took on the elaborate manner of little girls when they are mimicking one another. "Jus' wanted to feel that girl's dress--jus' wanted to pet it like it was a mouse-- Well, how the hell did she know you jus' wanted to feel her dress? She jerks back and you hold on like it was a mouse. She yells and we got to hide in a irrigation ditch all day with guys lookin' for us, and we got to sneak out in the dark and get outta the country."
They were both so unnerved by their near-death experience at the hands of an infuriated lynch mob that they traveled all the way to the Salinas Valley before taking another job.
In Chapter Two, George tells the boss that their work in Weed consisted of digging a cesspool; and in Chapter Three, George tells Slim about the incident that made them get clear out of the region, adding some further pertinent details:
I was jus' a little bit off, and I heard all the yellin', so I comes running, an' by that time Lennie's so scared all he can think to do is jus' hold on.
It is significant that George did not actually see what was happening until he heard the girl screaming. Since he had no opportunity to talk to anybody else while they were being chased and while they were hiding, he only had Lennie's version to go by. Lennie hardly ever tells George the truth. When Lennie kills Curley's wife in the barn in Chapter Five, George does not see what actually happened there either. In fact, only the reader and Lennie know what hapapened. George assumes that Lennie killed Curley's young wife while trying to commit rape, and he guesses, rightly or wrongly, that Lennie had a similar motive when he began molesting the girl in Weed. The combination of the two incidents make George think that his retarded friend is becoming a danger to society, a potential serial rapist and killer, and the combination of parallel incidents make him decide to kill him.
In Chapter Five when George first sees the dead body of Curley's wife in the barn:
"I should have knew," George said hopelessly. "I guess maybe way back in my head I did."
One of the things George is thinking he should have known is that Lennie wasn't telling him the whole truth about his attraction to the girl in Weed. If there had only been the death of Curley's wife, George might have tried to help Lennie escape the mob in order to turn him over to the authorities, hoping Lennie could be institutionalized rather than executed for murder. This is why the Weed incident is so important in the story. It helps to explain George's motivation when he kills Lennie at the campsite where the story began.