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The ticket in the bin
The single most common way a jackpot is lost is that somebody threw it away.
Winners have retrieved tickets from kitchen bins, from car footwells, from recycling that had already been collected, and in at least one case from a landfill after a council agreed to let them search it.
The reason it happens so often is that people check the numbers badly — glancing at a headline figure, seeing an unfamiliar sequence, and binning the slip without checking the secondary tiers.
The other common version is checking the wrong draw. Games with midweek and weekend draws, or with a separate raffle attached to the same slip, produce a steady trickle of people who compared their numbers against the correct night's results and the wrong game's.