With every scene the motorhome is in, the audio being played signifies it is powered by a diesel engine and has air brakes. There are numerous scenes in the movie when you can see the V10 emblem on the front corner of the motorhome. The motorhome is actually a Ford chassis with a Triton V10 engine and has hydraulic brakes.
Casey doesn't take out her iPhone to record Kenny's spider bites until a moment before he collapses, yet the video she posts on YouTube includes David and Rose convincing him to pull his pants down and show them the injury and you can hear David clearly say, "We can't stop!" all of which happens long before he collapses.
When Dave is making his rounds, dealing to his usual customers, his first stop is a woman with a baby. As she counts out what he's due the baby barfs up copious amounts of orange goo on his grey hoodie. At all subsequent stops his hoodie is perfectly clean with no sign of ever having been barfed on.
When the Millers & Fitzgeralds are playing Pictionary Melissa draws a little stick figure with a hat before the camera cuts to Kenny for his line. When they cut back to Melissa the stick figure with the hat is missing.
When David first approaches Rose about the Mexico trip deal in the strip club and she walks up to him, she has no money in her waistband. When she turns him down and walks away she has two bills tucked in her waistband.
When David and the family is about to cross the border back into the U.S, the guards find a joint in the hippie's van, one of the guards shuts the front passenger's door in one shot, but in the next shot of the van the door is open.
When David and Rose are talking on the plane (after Rose first boards), the camera is behind David and it shows they are standing by row 25. However when the camera switches to behind Rose they are standing by row 29. The camera then switches back to David and he is at Row 25. This continues until they go to the end of the plane for a family meeting.
When the Millers are at the Mexican border coming back into America there is one shot where there is a view from the front of the RV and the pot baby is in Rose's lap. Then she shot changes back to the interior of the RV and the pot baby falls into her lap from the overhead.
When the Miller family is about to enter the United States and they put on the sombreros, the chin strap on Casey's hat goes from behind her head, to under her chin, and when they exit the RV, it is behind her head again.
At the end of the film when David is in Brad's office, he says that all the weed is accounted for, but earlier in the film, the bag of weed that the Millers were pretending was a baby is run over by a semi. Therefore, it couldn't all be accounted for.
The Millers' RV is being towed with only a chain, so someone would have to be in the towed RV to apply the brake when necessary (e.g. on down grades, or when the lead RV brakes) or the Millers' RV would roll right into it.
As they reach the border checkpoint on their way out of Mexico the Fitzgeralds pull up next to the Millers in their RV and Don Fitzgerald rolls down his window to talk to the Millers despite the fact that the Millers have an RV with rubber-sealed passenger side windows that cannot roll down or even open. The RV the Fitzgeralds drive also has a permanently sealed rubber-gasket passenger side window, yet their RV window is simply gone when Edie Fitzgerald talks to the Millers by the roadside where the Millers' RV is broken down.
How did Rose get on the flight to Tucson? Assuming she was able to go back to the strip club and retrieve the plane ticket David bought for her, she would have had to buy a new one with her real name on it.
At the start of the movie, David is watching YouTube videos. When it displays the laptop, the video is playing but the pause/play button displays 'Play' which would only be seen if the video was paused, however the video is clearly not paused.