Throughout the series some Officers are shown on both the day, evening and Night shifts in a short period of time, some even within the same day. The Baltimore police department rarely gives shift changes until the next fiscal year.
Throughout the last two seasons, Carcetti repeatedly refers to a possible gubernatorial challenge in 2008, after serving two years as Baltimore mayor. But Maryland holds gubernatorial elections in off years - 2006, 2010, etc. The new governor would have been elected the same year that Carcetti was elected mayor - 2006 - and up for re-election in four years, not two.
Despite taking place at various times throughout the year over a five year period, there are only two scenes when there is snow on the ground. This is despite Baltimore averaging 20 inches of snow annually.
Despite being set in Baltimore,MD, only a tiny fraction of the characters (regardless of their race or ethnicity) have actual Baltimore accents. And this despite the characters clearly stating that they were born and raised in the city.