On the evening of Sunday 26 August 1928 May Donoghue, née M’Alister, boarded a tram in Glasgow for the thirty-minute journey to Paisley. At around ten minutes to nine, she and a friend took their seats in the Wellmeadow Café in the town's Wellmeadow Place. They were approached by the café owner, Francis Minchella, and Donoghue's friend ordered and paid for a pear and ice and an ice-cream drink. The owner brought the order and poured part of a bottle of ginger beer into a tumbler containing ice cream. Donoghue drank some of the contents and her friend lifted the bottle to pour the remainder of the ginger beer into the tumbler. It was claimed that the remains of a snail in a state of decomposition dropped out of the bottle into the tumbler. Donoghue later complained of stomach pain and her doctor diagnosed her as having gastroenteritis and being in a state of severe shock.
On 9 April 1929, Donoghue brought an action against David Stevenson, an aerated water manufacturer in Paisley, in which she claimed £500 as damages for injuries sustained by her through drinking ginger beer which had been manufactured by him. The case was ultimately settled out of court and the facts were never established in a court of law.
The identity of Donoghue's friend is unknown, but that person is referred to as "she" in the case reports (including the first paragraph of the judgment of Lord MacMillan in the House of Lords). Other factual uncertainties include whether the animal (if it existed) was a snail or a slug; whether the bottle contained ginger beer or some other beverage (as 'ginger' in Glaswegian and West of Scotland parlance referred to any fizzy drink) and whether the drink was part of an ice-cream soda.
source:
http://en.wikipedia.org/wiki/Donoghue_v_Stevenson