3 Indiana Jones and the Temple of Doom -- Mola Ram Tries to Get Back the Magic Stones (by Flooding Everything)
Mola Ram is the high priest of the Thuggee cult, a bunch of Indian dudes who hope to rule the world by gathering five lost magical stones. Mola already had three of these stones in his possession, but then that pesky Indiana Jones and his sidekicks stole them and jumped into a mining cart, escaping into a Donkey Kong Country level. Mola sends some henchmen into the tunnels to kill Indy and retrieve the stones so that he can finally reshape the world in his ugly image.
"... and Sinead O'Connor shall be my queen!"
How He Tried to Screw It Up:
At the end of the movie, Indy knows that the evil cult is thwarted because two of the Sankara stones wind up falling into the river and are lost forever -- that's how we know that the movie is over and the good guys won. So, sinking the stones under thousands of gallons of water is obviously against Mola Ram's best interest ... and yet that's exactly what he tries to do during the cart chase. It's like he was trying to end the plot half an hour earlier.
"I'll eat the heart of anyone who doesn't admit Raiders was better!"
See, henchmen isn't the only thing Mola sends into the tunnels: He also breaks down a huge water tank and floods the tunnels in order to drown the heroes, ignoring the fact that Indy still has the stones in his possession. If his corpse ends up buried in a vast, labyrinthine, watery tomb, how exactly does Mola intend to ever get the stones back?
Keep in mind that this is India in 1935, so we doubt that the Thuggee had lots of high-tech equipment on hand to drain the water out of the mines or could send scuba divers to search for small items. It would be quite a daunting task, and since Indy just freed all of Mola's child slaves in a previous scene, Mola Ram's now a little short on free labor.
"We're jumping ship to Nike."
Indy and his pals flee from the rushing water by escaping through a tunnel, which leads them onto a ledge overlooking a long drop into the river. Had they not escaped, the rushing water could have easily propelled their bodies and the stones into the river, and Mola's most prized possessions would end up in a crocodile's rectum.
The worst part is that the "flood everything" approach completely invalidates Mola's previous, far more sensible plan, which was "send henchmen after Indy." If these guards had managed to catch up with Indy, kill him and recover the Sankara stones, they would have had around 20 seconds to feel victorious before running into the huge wall of water coming after them.
"THIS HAT DOES NOT REQUIRE RATIONAL PLANNING!"
And speaking of classic Harrison Ford movies with inept villains ...
2 The Fugitive -- The Bad Guy Helps Harrison Ford Figure Out That He's ... Well, the Bad Guy
In The Fugitive, Dr. Charles Nichols is a high-ranking employee at a pharmaceutical company currently developing a new wonder drug. When Dr. Richard Kimble (Harrison Ford) finds out that the drug causes liver damage, Nichols sends a one-armed killer to murder him, but he winds up killing Kimble's wife instead. This works out for the best, though, since Kimble is wrongly convicted of his wife's murder and sent to death row. All Nichols has to do now is sit back and wait for Kimble to fry.
Or get frozen in carbonite, depending on state law.
How He Tried to Screw It Up:
When Kimble escapes custody and becomes a fugitive, he asks Nichols for help, since he still thinks of him as a friend. Instead of saying, "Yeah, sure thing, buddy, why don't you step into the guest room and have a nap?" and then calling the cops, Nichols ends up serving him the evidence of the conspiracy on a silver platter.
More specifically, Nichols sends Kimble to obtain incriminating tissue samples, even though they contain evidence that he himself was involved in the coverup. This, along with the fact that every lab member turns out to be inexplicably eager to help out a convicted murderer, allows Kimble to clear his name.
Jane Lynch has always had a thing for bad boys.
Kimble finds out that the tissue samples were fudged to cover up the whole "kills your liver" thing, and since half of the samples had been approved by a guy who was recently murdered, it was obvious that his signature had been forged by the only other person who had access. Namely, Dr. Charles Nichols. Both Kimble and the cops reach the same conclusion.
Instead of sending Kimble to the lab to collect the samples, Nichols could have volunteered to get them himself, which would have made sense from Kimble's perspective, since he was a wanted fugitive.
Or how about "Sorry dude, helping fugitives is a crime"?
Also, not to discriminate against people with disabilities or anything, but is a guy with only one arm really the most effective and least conspicuous-looking hired killer he could find? Did Nichols hire him over the phone, and then didn't want to say anything for fear of being rude? Maybe that's what happened with Kimble, too. Nichols didn't want to help him figure out the conspiracy, but just couldn't say no to a pal.