Most likely with older Champagnes that have been in storage a while since purchase, it is not that uncommon that a bottle is bad, has gone off or, as many will be familiar with, corked. More often than not, these references will be towards the performance level of the wine and will be down to poor storage, drinking past the recommended window or a genuine reaction with the cork.
Most Champagne that is produced annually and released to the market is recommended to be enjoyed sooner over later (non-vintage Champagne). Such wines will likely under perform the older they become. The Champagne wine-maker themselves will say that these wines are not meant to be aged.
Once a Champagne is released for sale, at this stage it will be disgorged, rested the required amount of time in the Champagne house cellars and made it's way to your hands, it will be aging and doing so open to the elements.
"Aged Champagnes, obviously especially vintage labels, will come with an estimated drinking / performance level recommendation. The big houses, the famous brands, such as Dom Pérignon or Cristal, will have easily accessible and frequently updated performance window graphs so you can see exactly when the 'experts' suggest they should be enjoyed for maximum enjoyment. Smaller grower Champagne houses may suggest this verbally and have less research / experience to give accurate performance graphs." Christopher Walkey - Founder Champagne Collectors.
To be referred to as gone bad or gone off, the Champagne bottle will have had to have been exposed to the worst storage conditions of heat and strong day light. It will likely have had a dried out cork with escaped wine and, even worse, oxidisation. Though still safe to drink, it will likely be flat, lost all fruit and floral characters with a damp cardboard taste and probably give you an upset stomach if you over indulge.
Visually, such Champagnes will show:
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Lower wine level inside the bottle.
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Misty / dark brown colour of the liquid inside the bottle.
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Damaged cork area with signs of seepage.
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Damaged or missing label.
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Damaged or missing neck foil or worn away foil showing signs being handled frequently - Was it a display item maybe?
Though it is estimated at a low 1-3% of bottles that are effected, a corked Champagne can also happen and effect the performance levels - Such wines are likely to be refused if served at a restaurant or refunded if purchased via a wine shop. Referred to as the cork taint effect, it's appearance is due to a chemical compound 2,4,6-trichloroanisole that comes into contact with the (natural) cork. Usually for a more senior wine professional / wine connoisseur to spot, the wine in the glass will likely show once again a wet cardboard character along with an unpleasant mushroom / mould presence.
