Another great Joni Mitchell song which a lot of folks think is a CSN&Y song -- 
Woodstock. 
 Joni Mitchell was supposed to have played at Woodstock but as the crowd size increased, her management grew concerned she'd be stuck there and wouldn't be able to make a very important appearance on the Dick Cavett Show, so it was decided she'd cancel from Woodstock.  Joni wrote Woodstock having not been there.  
Woodstock comes in two flavors:  
Joni Mitchell-Woodstock (1969 Live) which is the original folk song as Joni wrote it -- and then there's the CSN&Y R&R version tweaked by Stephen Stills -- 
Crosby, Stills, Nash & Young - Woodstock  Two totally different songs... I actually prefer Joni's (dark purple) version.  
If you're interested, the story of the song Woodstock as well has the CSN&Y song Our House is at:  
Graham Nash interview