This is the complaint I just sent to Red Bull. I will update if this actually works :

"To Whom It May Concern:

I am writing to inform you about a recent purchase of an 8-pack of Red Bull that was unsatisfactory to say the least. I am a regular consumer of your product so I am familiar with the taste. However, this particular batch left me regretting my purchase. As soon as I opened the first can and went for a sip, I noticed a pungent and unpleasant odour. I checked the cell by date and it was fine so I took a sip. It tasted far more sour than usual and left a nasty metallic aftertaste. I thought maybe it was just the one can but after trying the others, they were all of the same quality. I also got my friend to try it to confirm that it wasn't just me and he agreed that it tasted horrible. I'm just writing this to inform you that a reputable brand such as Red Bull should probably take more caution when it comes to quality assurance. This whole experience has deterred me from making future Red Bull purchases any time soon and has left me dissapointed with the quality of a product that I was looking foward to."