Episode #3.7 - S3-E7
Stupidity: Humphrey picks the gun from its case using a pen to avoid leaving prints (since for some reason he's not wearing gloves), but then starts messing with the box with his bare hands without having it dusted for any print. Since the box was owned by only one person and hidden under his bed, any other print would have been important evidence. (00:11:40)
Episode #3.6 - S3-E6
Stupidity: The villain concocts a demented plan that puts him at absurd risk (if anything goes wrong, he is caught red-handed) and accomplishes nothing, since it does not make him and his accomplice unlikely suspects any more than anyone in the group. It does not make it look like it was an outsider doing it (no attempt is made to fake a robbery), nor a result of an accident. It makes obvious it was a murder (a knife plunged in the back!) but does not pin the guilt on anyone else in the team, which would have been really easy to do (plant the knife or any of the victim's belongings in anyone's tent) and with the two being in on the scheme without anyone suspecting they were connected to each other, it would have been easy to create an alibi for each other, but they don't have any. It should also be noted that there's no reason why the victim wouldn't have exposed the culprit's crime earlier to his fellow birders, since he knew he was going to harm the animals.
Episode #3.2 - S3-E2
Stupidity: Throughout the whole episode, no effort at all is made to find out where were the chocolates purchased, and the bottle of champagne that accompanied the box(es) is completely ignored, not even addressed in a throwaway line (such as that it's too common to trace, or whatnot). It's not directly part of the murder trick so it not even considered. The explanation of why the other incriminating box is still there is also quite ridiculous, since the murderer is left alone for enough time to do strike again with a very timely effort involving her leaving the only room of her bungalow (which involves her, the most known and conspicuous person on set, stalk her in plain view for an undisclosed amount of time), but could not find a few seconds to take a walk a few feet away and bury the box in the sand or bushes, flush the poisoned chocolates down a toilet, throw them at sea, just about anything. It's a bit of chocolate and cardboard, not exactly hard to dispose of that in a big set in the wild.
Episode #3.7 - S3-E7
Stupidity: The detectives suddenly bring everything to a dramatic halt to have a word with Joseph, because Fidel has been able to check the prints and his were the only ones on the gun and therefore he became really suspicious. It's his own gun, and people can wear gloves - in fact they do all the time! The fact that they automatically turn on him and he is unable to defend himself is simply 'because the plot this time says so', since in every other episode Joseph's prints would have been exclusion prints - they are supposed to be there and there's nothing suspicious about it.
Episode #3.6 - S3-E6
Stupidity: At Catherine's, Humphrey is browsing the photos; neither he or his agents noticed before amongst all the bird and wilderness pics the very obvious series of shots of a woman in a hotel poolside and in close-ups. And yet he has to have printed the (obviously enormous, since he already went through them in the afternoon and investigated the cane seen in them) amount of photos entirely, but finds the photos dining outside from a very small pile on his little dinner table. Those pictures stand out at first glance.
Episode #3.2 - S3-E2
Stupidity: Spoiler - In the flashback, apparently the killer 'framed' Susie by going as herself and on the phone going "Helloo, Susie speaking" next to the one guy that is a police informant - not the guy who sold her the fish poison. It would have made sense if she did that while she was buying the tetrodotoxin, but there's no way she could have known that that particular guy was going to talk to the police and they wouldn't be able to instead get a description from the original poison seller (that the police seems to ignore anyway and just let him deal unscrupulously - nobody has a good reason to own a bag of venom - in deadly substances).
Episode #3.2 - S3-E2
Stupidity: The whole case could and should have been solved in 5 minutes if only the cops did what any normal cop would have done; once it became clear (very early in the case) which rare poison was used; trace where it came from. Which in this episode is depicted as being ridiculously easy (all it takes is send Dwayne to the docks to give 20 bucks to his buddy), and is instead resolved very late because it is relegated to a little side-quest Fidel has to deal ineffectively with while the main characters psychoanalyse people and air their dirty laundry.
Answer: Most likely they had only tropical exotic drinks, he wanted an old fashioned English beer.