Stupidity: When Clark is trying to sneak Lana out of the house, they walk past the front door in order to go through the kitchen door, where they get caught. It makes no sense to not use the front door. This way Lana is already outside, the whole goal in the first place, should his mom or dad see them. Not to mention the fact that Lana's car is still parked outside and Johnathan or Martha would have seen it, so they shouldn't be surprised to see her in the house.
Stupidity: The last remaining Xindi, in desperation, attempts to release the virus over the fan, despite the virus not being fully ready. However, it seems the only reason he didn't release the virus at that time was he had to seek cover from Archer and T'Pol shooting at him and to start shooting back at them. However, he had plenty of cover from the rooftop stairwell, which was right next to the fan, and he had enough time to dump out the contents over the fan.