Bad Design
I've never liked the design of Diet Coke's citrus line extensions-- Diet Coke with lemon and Diet Coke with lime--because they look way too much like the real thing. More than once, I've bought the wrong kind. Amazingly, I didn't this time. Look closely (click the photo for a full-size version):
The second row from the left is regular Diet Coke, sandwiched between rows of Diet Coke with Lime. The green cap is part of a promotional sweepstakes.