This question already has an answer here:
Can anyone please tell me what these two parallel lines on my exit stamp mean?
And will it impact my further application & travel to the Schengen area? It happened when I was denied entry to Croatia due to single entry Schengen visa and therefore I have no valid visa to enter Croatia. The Croatian border guard didn't stamp anything on my passport nor gave me any letter, she just kindly told us we can apply for a visa in Ljubljana. The Slovenian guard just drew two parallel lines on our exit stamp and when we left the Schengen area via Switzerland the country which issued my visa the guard just stamped me out without a single question.