One letter is missing in surname - how to solve this in document verification?

1, Is your name correct on all your certificates? Then I think there will not be any issue. They might have asked for clarification before calling for the examination and interview. It can be treated as a typographical mistake and ignored by the authorities.
2. If there is a difference in any certificates, you may have some problems. Then you may have to apply for correction through the authorities.
3, In any case, a court affidavit will serve the purpose. To be on the safe side, contact any lawyer in your area and ask him to get an affidavit and keep it with you. When you go for certificate verification you can show that affidavit and tell them that the correction process is already initiated.
4. These days if the correction is there in any of your certificates, you have to get a Gazette publication. This process starts with an affidavit, then publishing the same in two news papers and then applying for gazette publication. This process also will be known to the lawyers and you can get their help.

As mentioned above, there would not be a problem with any kind of test. Generally, It is considered a typographical error, and If you are afraid that they would reject you while document verification. So, you contact a lawyer and ask him to make an affidavit. And show it during the document verification and ensure them that the correction process has started. After that publish it the same in two local news papers and apply for gazette publication.

Whether it is one letter or more letters any change in name is to be rectified otherwise there will be mismatch during the document verification session. This is a serious matter and it has to be rectified at an earliest.
What you have to do is that make an court affidavit by taking help of some lawyer and mention all the relevant details about the mistake made in the application form. Attach copy of all the documents with the affidavit making references on each page. You will have to sign in the affidavit and it will be countersigned by the magistrate. Copies of all the supporting documents would be attached with the affidavit and would be its part only.
Now keep a copy of the affidavit as ready reference with you and take it when you go for document verification process. You have to keep the original documents also with you so that if the verification authority wants to crosscheck then it can be done easily.