As Mahi and Raj sample the chocolates in the Swiss candy shop the chocolates at the corners where the shop keeper takes the samples from keeps getting replaced when the camera angles change.
When Gayatri rejects Raj's wedding proposal, he goes through a flashback of how he had betrayed Mahi and Radhika before. In the flashback, he sees Radhika crying at the place where they were supposed to meet for their wedding. He was already on a flight to Sydney when that happened.
Raj works at Microsoft in Sydney as a game developer who made Halo 3. But Microsoft was just the publisher of Halo 3. Halo 3 was developed by Bungie Studios.
While playing in Swiss Alps, Ranbir and his friends are heard singing "Dhoom Machale". The story in this movie revolves around 1996 and Dhoom wasn't released until 2004. However, background music is rarely dictated by which time period the film is in.
During the party scene in Raj's house, the song that is being played is Jhoom Barabar Jhoom, but then neither the song nor the film had released when this story was supposedly taking place.
In the scene where Mahi misses the train, she is shown reading Harry Potter & The Half-Blood Prince. Even the first book in the Potter series wasn't released in 1996.