(Deliberate by filmmaker) During the pool scene, as April enters the water is played by Lindsay Lohan. After Machete enters the pool, the character is clearly not Lohan. This is because the footage is actually from the "Machete" fake trailer that preceded Robert Rodriguez's Planet Terror segment in Grindhouse - one of several pieces of trailer footage that were re-used in the movie.
When Booth is driving to pick up Machete at the day labor site, he is seen driving a Mercedes Benz S-Class (W220 1999-2006). At the day labor site, he is seen in a Mercedes Benz S-Class (W116 1972-1980). When he drives away, the car switches back to the W220 model.
In addition, in the inboard scene at 0:16:36 you can see the headrest of the W116 again. Later on (e.g. when Michael Booth gets his daughter from the house), the headrest is one of a W220 model.
The window next to the door in Luz's house changes between shots. In the exterior shot it's covered with cardboard or some other brown sheet (we even see the explosive bounce and fall to the ground) while from the interior it's a normal window with curtains.
During the monologue leading to the showdown between Torrez and Machete, two actors can be seen fighting behind and to the left of Torrez. During the dialog, they can be seen in and out of frame multiple times.
In the opening scene where Machete and his partner are attacking the drug-house, it is very clear to see both his partner get shot from the front, and the windscreen take a few bullets, as they approach the building. However, when the car crashes through the building, the windscreen is completely intact and bullet-hole-less.
In the beginning, when Machete pulls his machete out, it appears to have a ring near the hilt (Machete uses it to swing the weapon like a windmill). Later on, when he rescues the girl and she sticks to him, the machete has no ring in the handle.
When Machete is ambushed in the motel room, he fires five shots from his revolver at the two men coming in through the front, then shoots a third man to his left three times, meaning he fired eight shots from a six-shot pistol.
When the senator is shot at the beginning of the film, it clearly happened in Austin near the capital. When the policemen are talking about which hospital he'll be going to, they mention Parkland, which is in Dallas, 200 miles away.
Lindsay Lohan shoots the senator point blank three times in the chest with a .44 Magnum. Hours after the attack, the senator reveals a bulletproof vest and no real injury. However, at point blank range, a .44 bullet has enough impact to cause serious bodily injury.
When the video of Senator McLaughlin shooting the immigrant is shown to the media, it plays as if it's been shot from three different angles (the same as the original scene from previously in the movie) instead of the single camera the thug was actually recording with.
When the body hits the car with the thermometer in his neck after the house explodes, as the car goes into reverse and the body falls off the front, you can clearly see the stuntman move his arms up to protect for the fall to the ground.
When Senator McLaughlin (Robert De Niro) carjacks the taxi, the registration sticker clearly expired in 2008. Most of the other cars stickers in this movie didn't expire until 2010. This means the taxi would have been operating illegally.
During the shootout at the church, Tom Savini's character is holding a gun that tends to shoot like an automatic rifle (eg M-16). However this gun is a USAS-12 automatic shotgun with a 10-round-capacity box magazine. You can see shotgun shells ejected while watching in slow motion.
When Machete impales Torrez in the stomach with his machete, it is initially seen exiting out his back. In subsequent shots the machete is still stuck in Torrez up to the hilt only no blade is exiting.