In "The Pit and the Pendulum," did General Lasalle's arrival seem an exceptionally lucky coincidence to you?
Yes. The way that General Lasalle arrives at just the moment before the protagonist plummets into the pit of flames that threatens to swallow him up clearly indicates that this is meant to be seen not realistically as a believable occurrence: Poe obviously means General Lasalle's arrival to stand for something more significant than merely the incredibly fortunate deliverance of the protagonist. Note how the final paragraph of this short story describes his advent:
There was a discordant hum of human voices! There was a loud blast as of many trumpets! There was a harsh grating as of a thousand thunders! The fiery walls rushed back! An outstretched arm caught my own as I fell, fainting, into the abyss. It was that of General Lasalle. The French army had entered Toledo. The Inquisition was in the hands of its enemies.
The reference to the sound as being of "a thousand thunders" and the "blast as of many trumpets" gives this sudden rescue almost apocryphal overtones, suggesting that this story with its ending is meant to be read allegorically. The trumpets are an allusion to the trumpets that are mentioned as heralding the end times in the Bible, indicating that the character of General Lasalle is meant to be read as a saviour figure who has saved the narrator's soul from plummeting into the depths of hell. The ending is therefore meant to point to the allegory within this tale and the different way in which this story can be read. General Lasalle's arrival is of course unbelievable, because of the allegorical overtones of the story.