Stupidity: The plan to infiltrate Emma into the coroner's office in a body bag is stupid. She is wearing a paramedic's uniform, and we see later when Mickey arrives, nobody is checking ID papers. How would she know that there was nobody in the room when she emerged from the body bag? If she could walk about the building without being stopped, why couldn't she simply walk in through the wide open front door in the first place?
Stupidity: When Mac and Lindsay are reconstructing the stab wound found on their victim on their pig specimen, a couple of the weapons they use are long, thin knives; since they obviously don't match the circular wound, Mac at the very least would know those could be excluded by default.
Stupidity: Characters repeatedly make note of how hard to kill the Wraith are, but sometimes, especially in the final episode(s) of season 1, they go down easily in a few bullets each, and then don't even regenerate as the characters turn their attentions elsewhere.
Stupidity: Chazz returns two cards to his deck and redraws another two cards using Magical Mallet. Alexis appears to recognise the two cards (Armed Dragon LV10 and White Vale) calling them "Two very powerful cards." Chazz then summons Armed Dragon LV10 and equips it with White Vale. Alexis attempts to activate a trap, but is surprised to find out she can't, to which Chazz replies the White Vale protects his Armed Dragon, but if she already recognised the card, then she should be aware of this. (00:17:35 - 00:18:30)