I'm rather particular about settings and timelines. Unless all of the action takes place over a weekend, people will age have birthdays, anniversaries and there will be particular newsworthy events.
In Running Home I needed a credible reason for killing off one of the bad guys and decided that it would be best if he was killed in one of the Hezbollah-Israeli conflicts. It took several days to research but I eventually decided to set the book in the summer of 2006.
When I lack the creative energy to write a new chapter, I will often review and edit previous ones. My current project, The Governess' Daughter, is primarily set in London during the fashionable Season of 1817. I discovered two anachronisms that bothered me enough that I restructured or rewrote certain portions of the draft.
The main female character, Phoebe, is a very capable scholar and biblophile. Therefore, her suitor, Viscount Briarly, offers to take her to the library at the British Museum. The original scene has Briarly introducing her to the chief librarian while she is awestruck by the magnificence of the Reading Room. The problem is that although the library existed in 1817 and was housed at the time in exceedingly cramped quarters at Montagu House on Great Russell Road, the famous Reading Room was not built until 1857.
Similarly, Viscount Briarly, in common with many of the nobility in that era, is a cricket fanatic and a member of the Marylebone Cricket Club which played at the new Lord's cricket ground in 1817. I decided that Briarly and his best friend would spend the afternoon watching a first class match from the storied Long Room in the main pavillion. Unfortunately, due to the chilly weather and a scandal ridden series of matches at grounds outside London, first class cricket was not played until July at Lord's that year. And the grand pavillion wasn't actually built until 1889, so I had to settle for them eating at the Tavern and knocking a few balls around.
There are another dozen or so things that I've discovered. For instance, inheritances were disputed in the church courts and the lawyers there were called proctors. Railways had been in use for decades in various mining districts but they were effectively reinforced wagon ways which allowed horses to pull heavier cargos with less effort. Most of the notorious scoundrels of the Napoleonic war era (Byron, the Shelleys, Beau Brummell, etc.) had already left England because of insolvency or health or boredom making it difficult for my villain to find a good house party dedicated to his kind of debauchery.
On the other hand, I have learned a great deal about daily life in the era. And I would have rewritten everything a dozen more times anyway.