Ahm... I see that a few users had made something like this, so I decided to make one as well. Note that it isn't in order.
0.5. Here we go!
1. Zombies can eat Peashooting plants (excluding elemental Peashooters), then why can't they eat the peas fired by them?
2. Because the travelling speed of the Peashooting plants' peas are considerably slow, then why can it make it to the zombie without falling due to gravity?
3. (PvZ2) Why do the peas come from its root, through its stem, and go straight in its mouth?
4. Suns are incredibly hot, then why can Sunflower produce it without burning herself, the other plants, and the lawn?
5. Cherry Bombs when exploding would leave craters and stains as well, because it is a bomb.
5.5. The zombies' ashes would leave stains as well.
6. Where on Earth did Wall-nuts disappear when it got eaten? Even during its second degrade (third in PvZ2), it is still a very big piece of walnut, and zombies can't swallow that much, considering about the size of their mouth and their stomach.
7. Zombies turn blue when it got hit with an ice object? #FrozenPlantsHaveFoodColoring
8. Doom-shrooms would destroy the whole lawn, due to nukes in real life are dangerous.
11. Potato Mines now defies the law of physics in PvZ2! It can be planted on Lily Pads!
12. How can Bowling Bulbs get their bulbs out of the ground? Real bulbs have roots.
12.5. Furthermore, it can be planted on Pirate Seas plank and on Lily Pads!
13. The flame inside Torchwood would've burn it.
13.25. Furthermore, the flame would go strong (due to "eating" woods) and eventually burns the whole lawn.
13.5. The fire inside would've been extinguished, or at least generates smoke when a frozen pea pass by, due to the water it generates when the ice melts.
13.75 It would've burn peas instead of igniting them.
13. Jalapenos don't burn the lawn and the other plants? Jalapeno Zombie did burn plants, but they didn't burn the sod...
14. How can Pea Pods' peas ignite when some passes under the flame of the Torchwood?
15. Hunter Zombie is a vegetarian right? Why does it eat brainz?
16. Pharaoh Zombie's sarcophagus.
Crazy Dave: I have a fan!
17. The Zombot can stand on the roof without breaking it, or at least leaving cracks?
18. How can Magnet-shrooms don't push, or attract, each others?
19. For God's sake, medieval times have mailboxes?
21. Zombies don't need air, then why do Ducky Tube Zombies need an inner tube?