When the rental car rolls down the hill, it lands on its roof. When it explodes, it is right-side up again.
While riding with Oscar down the road, Felix states that they need to pull over so he can eat, because his blood sugar requires him to eat every four hours. Earlier in the drive, though, he says "I'm starving, I haven't eaten since last night." That would mean it's been well over four hours - closer to 16-20.
Just before getting into the rental car, Oscar is carrying both his and Felix's luggage. His own suitcase in his right hand and Felix's in his left hand. Oscar steps down a small staircase at the rent-a-car office and the suitcases switch hands in an instant.
In the scene where Felix and Oscar are in that old man's car, and the old man dies, it is clearly visible in the following shots that he is breathing.
While Oscar and Felix are driving down the road, the ignition key is in the off position.
The first FOUR letters in Felix and Felice are the same, not just 3.
When Felix goes to open the trunk of the car with the key to look for a new map and then can't find his shoot case he tells Oscar to open his window, so he does. Then Oscar opens his door to go find Felix's suitcase, and there is a beeping sound. If the keys weren't in the ignition then Oscar wouldn't be able to open his window, and you wouldn't hear a beeping sound, since Felix opened the trunk with the car keys.
While Oscar is eating a peach you can see that it is half eaten but when he tries to throw it out the window it is almost whole, and only slightly bitten into.
At almost 15 mins. During a scene where Felix and Oscar are driving on the freeway, you can see someone in a passing car take a photo of them.
The suitcase Oscar is holding on the movie's poster is not the suitcase he actually has in the movie.
When Felix and Oscar are called out of the holding cell, Felix says "If we go down, you go down with us" to the truck driver. As he says this it cuts between two shots quickly, and in the first shot when he says, "If we go down ..." his mouth isn't moving.
Felix and Felice claim to be meant for each other, as the first 3 letters of their names match. Actually, it's the first four.
Upon finally reaching the wedding in San Malina Felix is yelling at his ex-wife. He claims not to have seen her in half a century fifty years. Their daughter's got to be in her late twenties or mid thirties.