Passport and customs/immigration

I have a potential problem I'm looking for advice on or maybe I'm overreacting. I have a U.S. Passport and I've been to Japan once before and I was granted entry with this same passport with no trouble. A year ago for a domestic USA flight a TSA agent wouldn't let me pass because I messed up the signature of my last name in the passport. Right now it appears my first and middle name written clearly then my last name written over my mistake in darker writing pattern.

I'm going to Japan in a few months and after being on at least 8 different domestic USA flights I haven't been questioned about it. Will Japanese customs and immigration really deny you just by a signature mistake? It has the same printed info and the picture looks like me. I'm not sure what to make of this or what is it they look for.

This is just my opinion, not advice.
How old is your passport? If you are getting near to renewal it could be worth getting a new passport and taking care over the signing.

From an non-American point of view, your TSA people are some of the most rigid scrutineers
I've ever come across. I understand why, as your country has been the target of some of the worst terrorists. However, the TSA do pick up on some pretty petty stuff, or it seems that way to a non American like me!!
Can you contact your Internal Affairs Dept, or whoever is responsible for issuing passports and discuss it with them.

Somehow I don't think the Japanese immigration/customs are so pedantic as TSA but it could be a gamble you don't want to take.
Good Luck, attend it it sooner rather than later.

I was not aware one needed a passport for a domestic USA flight. Then again, I only take international flights, so I always need a passport.

I was not aware one needed a passport for a domestic USA flight.

Passports aren't required, but they are one of the acceptable forms of photo ID.

http://www.tsa.gov/traveler-information/acceptable-ids

To the OP, the easy solution may be to have your passport reissued.

Right now it appears my first and middle name written clearly then my last name written over my mistake in darker writing pattern.

If I were you, apply for a new passport.
You problem might start in your own country before boarding.
Prevention is better than "curse".

Correct I use a passport for domestic flights because it's an accepted form of ID. It's only 2 years old but I've been leaning towards applying for a new one rather than risk it. I had no idea a spelling error on my last name could lead to this. If I was a border agent and saw this type of signature error I wouldn't deny someone passage but then again I'm not one so I don't know how they scrutinize something like this. I had someone look at it but they said it's borderline of being ok and needing to get a new one issued. My trip is in October so I'll get on the paper work for it.

I had no idea a spelling error on my last name could lead to this.

Names not matching is something that they look out for (this can be a headache for dual citizens). And the way you've described your error, I think makes the entire document look suspect.

I don't understand the part of not writing your last name correctly on your signature...

So many people have a weird scribble as signature that no one can read, is hard to imitate, and is unique to each person.
At any rate done is done, get another passport. Don't they make you sign you passport on an electronic pad? if one make a mistake one can redo it..