When Walt shoots Mike, it goes from being the middle of the day to sunset in a matter of seconds.
When Heisenberg, Mike & Jesse meet Declan, the clouds behind Declan change from shot to shot. The apparent position of the sun (time of day as indicated by shadows) also changes.
Mike dumps a computer and guns down a well. The apparent position of the sun (time of day as indicated by shadows) also changes.
When Dan is in vault, the banker uses one key to open all the boxes using the right key slot. This is impossible since a safety deposit box needs the bank key and the box owner key to be inserted and turned simultaneously. Additionally, when Dan uses his key, he changes back and forth from the left to the right key slot among the different boxes.
During his pitch to Declan, one of Walter's arguments is "Higher purity equals a greater yield." Usually, the opposite is actually true. The yield is the amount of product you are able to make out of a given amount of precursor. Purity only indirectly comes into it: At the end of the synthesis, the raw product is contained in a mixture along with unreacted precursors, solvents, catalysts, byproducts etc. So as a final step, you need to separate it from all these other substances using a suitable (combination of) separation methods like filtration, distillation, etc. However, with each such step you take to clean your product, you inevitably also lose some product along with the unwanted substances you are removing from it. This means the yield goes down. You can take any number of steps to increase purity into ridiculous figures but in doing so, you would lose more and more of what you were actually trying to make. (By the way, this is the basic chemical flaw underlying the complete series' premise with the supposedly super-pure meth. Purity is just a crutch to avoid having boring exposition to explain all of this.) However, the point is that Walt is such a good cook that he doesn't need to rely on a "final step" to get rid of a bunch of unwanted stuff. He has the knowledge to keep the unwanted stuff to a minimum all throughout the process and can therefore do more with less, hence a greater yield.
All the $100 bills put in the safe deposit boxes have the same serial number: B32862220B.
When Walt begins to teach Todd how to cook, Walt is shown using plastic pipettes to sample the mix. Walt taps the pipette against the beaker and the sound of clinking glass is heard.
Mike has already been burned once by losing the off-shore account set up by Fring for Mike's granddaughter, Kaylee. Even if he had never heard about diversification in his whole life, a street-smart and cunning character like Mike would never make the same mistake twice and put the complete 'nest egg' into one single place again.
The DEA visit Mike's home with a warrant. Seen from the back Hank is readily identifiable due to his limp. Viewed from the front, Hank has lost his limp and swaggers into the hallway.