I had this happen ONE TIME and one time only.
20 gallons of paint to work with and had what rmicahel calls " working bucket" anyhow got down to the last 5 and as i'm pooring it in I see the color difference but thats like half a 5 already poored in so I check the label and sure as heck its a different color, and I needed every bit of that 20 gallons. I called the store mad as heck told them to delivery me a fresh bucket and NOW.
Always check the labels on stuff before you get going and I even check the tint by dabbing my finger in the working bucket or vice versa and check the tint in the other bucket just to make sure, wipe it on a lib or something .
Sometimes they forget to tint one had that happen also , but thats usually pretty obvious just that once you get going, if your like me your going like a madman spraying paint and it's all in your eyeballs
might miss something.