I'm going to Iceland with a single entry "Schengen States" Visa. On my way back to Egypt I'll pass by Frankfurt and Athens with flights. Will I face any problems regarding entering the other countries?
You have the right visa, so your paperwork is good. Any country or particular border agent may question you but your single-entry Schengen visa allows you to enter the Schengen area exactly once. Since Iceland, Germany and Greece are all part of the area, you will only enter at your first landing point. On your way back, flying from Iceland to Frankfurt and then to Athens keeps you within the Schengen area, so do not need more than a single entry visa.
- In principle, a single-entry Schengen visa allows you to enter the Schengen area once and to cross as many internal Schengen borders as you like. There are towns where one can easily make several dozen border crossings a day just by going for a walk.
- You explained your travel plans in your visa application and you should avoid the appearance that you misrepresented them.
If you arrive in Athens with a visa from Iceland and plane tickets to Frankfurt and Iceland, everything will be fine since you are obviously going to Iceland.
If you arrive in Athens with no concrete plans of onward travel, there would be questions.