Canny shoppers always check the dates on food before buying it, but in a rush you might not bother to notice the distinction between 'best before' and 'use by'. That could lead to some unpleasant results.
The NSW Food Authority has a useful summary of the terminology used on food labelling in Australia. The bottom line is pretty straightforward: products marked 'use by' a certain date shouldn't be consumed after that date. Unsurprisingly, that mark is mostly used on fresh foods. 'Best before', a staple of canned and packaged goods, provides a recommendation, but not an absolute guideline. (I've certainly eaten tinned goods that have passed the best before date, though it always pays to check the contents before mixing them into your daily gourmet delight.)
You'll often see supermarkets selling products with an imminent expiry date -- that's not a legal obligation, just a canny strategy to try and shift goods. Once they've passed a 'use by' date, they legally can't be sold at all, while there's actually no law against selling 'best before'-expired goods if they aren't damaged.