About 10 years ago there were some cases of poisoning which were traced to the use of a flavouring agent, diacetyl. This was only used in microwaveable popcorn and was supposed to taste like butter. Even so, you would have to eat a lot of popcorn before you had any problems (the cases mentioned involved at least two bags a day), and the flavouring is still legal although a lot of companies don't use it any more.
Apart from this, of course you can have an allergic reaction to popcorn - corn allergies are quite common - and the fibres can be hard to digest. Too much popcorn can also interfere with your body's ability to absorb vitamin B.
Mainly though, it's not popcorn itself that is the problem but the buttery, salty or sweet flavourings that people consume with it.
The 'popcorn diet' is not recommended, anyway - you can learn more about that here.