Are there any scams for teaching English in Japan? Well, I would say that there aren't many, yet they exist.
Scam is a strong word, yet here are some things that you may encounter.
Scam #1 Don't pay anyone for a job teaching in Japan or any sort of recruiting fee
You can if you like, but I wouldn't personally do that. There is no need to do that. There are recruiters that do charge fees and they are not always a scam, but more common is when the recruiter gets paid by the school. The bottomline is you don't need to pay anyone for any jobs in Japan.
Beware of any sort of fee prior to starting any job abroad.
Scam #2 Eikaiwa (private institute) scams
Now most eikaiwa will not scam you. But some people have poor experiences with them. Often what can happen is that the contract is not honored. Some say the bigger companies are safer, but remember the NOVA collapse in 2007?
Scam #3 health insurance
Most employers who employ you full-time will assist you with healthcare. If a school tells you that healthcare is only for Japanese people then that is not true. Learn more about this scam.
Scam #4 visas?
Some will tell you that you cannot enter Japan on a tourist visa and teach English. It is illegal to teach on a tourist visa, yet traveling to Japan on a tourist visa is okay. You can go to Japan and find work there. If a school says that you need to start on a tourist visa then some say that is a scam, but not necessarily. Maybe they are late processing or hiring a teacher. Getting a proper visa can take many weeks.
Long term employment on tourist visa is pretty unstable and if you get caught you will likely get kicked out, possibly have to pay a fine, etc.
If a school asks you to start on a tourist visa then it's possibly scammy, but not necessarily.
It's more of a grey area than a scam if you ask me. You don't have to enter Japan on a work visa and you can change your tourist visa to a work visa in Japan.
Scam #5 TEFL course scams
TEFL/TESOL courses can be a little a dodgy. There's a lot of fibbery out there in regards to these courses. There are outright TEFL course scams (example) and half-lies. Do your research and know what the requirements are.
Scams are often done by middlemen who take advantage of new naive teachers. These middlemen can be recruiters, TEFL course providers, affiliates, schools, etc.
Scam #6 The "ALT scam"
So you may have heard that ALT teaching is better than eikaiwa, but it depends. The JET program is a notable program that places ALT teachers in public schools in Japan that is run by the government. However, the board of education has been subcontracting its' work out to dispatch companies.
There are different dispatch companies but they all skim money off the top of the teacher's salary. If you happen to work directly with the BOE and find a job in a public school (which is rare) then you will make more money.
If you work for the JET program you will make more money.
The dispatch company gets hired because they act as a middlemen and do some of the hiring process. Some dispatch companies are better than others, but they all take money off the top so that's why some call it a scam.
Learn more about teaching English in Japan.