The first episode of Season 6 occurs 1 month from the end of Season 5, the death of Detective Jessica Angell (Emmanuelle Vaugier) and the bar the CSIs were in to toast her memory was shot up, resulting in Danny Messer (Carmine Giovinazzo) losing mobility in his legs.
Gloria Votsis (Risa Calaveras) appeared in the CSI: Miami (2002) episode Last Stand (2011) in a different role.
Mac quotes Shakespeare saying "tempt not a desperate man", this is from Romeo and Juliet (Act V, Scene III)