I had a similar experience where my mouth went numb from a 2003 MRE Pound Cake, when I was much less experienced with what to and what not to sample I tried the Cheese Spread from a 1996 BBQ Pork MRE, it seemed pretty normal besides the off-grey color and it tasted a bit stronger than the new stuff....

Spanish isotonic vitamin C drink... thing for me, sugar water is cool and all but it janks up my blood sugar. 90's MRE grape drink powder is a close second for me. The worst I've had by far is modern MRE fruit punch or really any modern MRE drink powder because of those pesky clumps.