That can happen if you're served Guinness that's too cold. Or the Irish couple have bought into the massive ad campaigns for lagers like Bud and Stella and Carlsberg that refer to those beers almost exactly as you've said they replied, compared to cask-conditioned "real" ales, porters and stouts.

Guinness is intended to be served and savored at around 45 degrees F. Here's a quote explaining why that I have in my notes from Ken O'Callahan, who's a Guinness draft specialist based in Chicago (his job is to travel around making sure the Guinness-serving bars in his region of responsibility are cleaning and maintaining their taps, correctly storing their kegs and properly pouring the pints, usually by inspecting the draft system then drinking some of the beer -- what a job, eh?)

"Once you get below 40 degrees you don't get the proper gas breakout. The gas stays in the beer so the customer gets filled up. It can lead to a bitter taste and creates a small head on the beer," O'Callahan says.

Perhaps the couple who found it "so heavy" weren't being poured properly tended Guinness. I wish I could say that seems unlikely in Ireland, but there has been some debate over there about Guinness being served overly chilled, with the corporate beancounters at Diageo (the conglomerate corporate parent of Guinness as well as Jose Cuervo, Johnnie Walker and Smirnoff) not caring -- some have said even encouraging it as a way to build market share against lagers -- so long as people buy the beer.