Communicative language teaching traps

Communicative language teaching is all about having the students use the language to communicate something meaningful. 
This isn't as easy as it sounds.  There has to be a genuine reason to listen or read what the other is communicating.  there has to be a goal that both communicators are working towards.  And, it has to be meaningful to the learners.

The first trap: SELLING
Trying to convince someone to buy something or to do something is an excellent idea.  However, there must be the possibility of rejection.  No prospective buyer can buy everything.  Even Bill Gates, arguably one of the richest people alive, doesn't just hand out his money to charities willy nilly.  He has them prepare a good pitch, which he evaluates and considers.  In the end, he donates his money where he thinks it will do the most good. 
If you're preparing some kind of a buy-and-sell activity, the buyers have to be limited in some way.  Money is the obvious choice; each student is given a set amount to spend which is less than the total of what is being sold. 
However, you can also simply say "you are only allowed to say yes to a maximum of three". 
A final way is to make the task real world.  For example, my students have to convince their classmates to sign a pledge to boycott a particular company for the duration of the term.  They can sign as many as they think they can reasonably handle.  Most students will honestly journal their attempts and failures in such a boycott.

The second trap: AWARENESS RAISING
A lot of teachers might have their students research a topic such as climate change or healthy eating and then make posters as an "awareness raising" project.  That's cute, but like all advertising, the target group needs to be carefully identified.   You'll do better making little storybooks for kindergarteners or facebook campaigns for teens tahn just putting up useless posters in the hallways.  Kids know when something is real and when it isn't, so make their work as real as possible.