My DS was staying recently and requested marmite on toast. On examination the marmite jar was inscribed 'best before June 2009'. Although it smelled normal for marmite and there were no visible signs of fungal growth, he refused to eat it and to my shame I capitulated and threw the offending jar away. I posted on Facebook about the incident and was roundly told off by several friends who were of the opinion that the flavour would become more exquisite as time passed.
Generally I ignore use by dates, except for fresh meat and fish, but after my DS and family had gone back to the States, I discovered some tubes of Yeo yoghurt in the fridge that DDiL had bought. By this time they were 2 weeks past the use by date, but I confidently presented one to Mr A for pudding. The tubes contained a horrible mass of separated curds and whey, with a sort of sparkling alcoholic flavour
Somehow the combination of sparkling and alcohol was not nearly so appealing as when it comes out of a large green bottle