While looking for evidence in Walt's bedroom, Vic picks up Donna's bra and puts it in an evidence bag. But at the moment that Walt's door was kicked in, Donna was still wearing it.
In several scenes, most notably when Vic & Walt are driving in Vic' s truck, you can clearly see Katie Sackhoff's real hair very distinctly in contrast to her ponytail extension.
When Cady is helping Walt in the opening sequence, there is a tight shot of Walt's bloody belt buckle, fully fastened. In the closing scenes of the last episode, Donna loosened his belt.
When Mathias supposedly drew blood from Henry he a) didn't even use a tourniquet and b) drew from a random place on the forearm. Blood draws are done using a large vein in the arm in the antecubital space. You can't just stick a needle anywhere for a blood sample.
Mathias shows Henry a .30 calibre slug in an evidence bag, and relates how it ricocheted off a rock and into Henry's "butt". He may have guessed by the blood on the seat where Henry was shot (it was his thigh), but where did he get the slug? Henry removed it himself when he was out of the truck at a random roadside location before passing out. Gab wouldn't have picked it up.