I met a person who told me that he worked in an embassy for a long time and that there is a difference between the places of the stamp.
For example, if you have a visa and your stamp is on the visa page, or in front of the visa page, it means that your visa is activated, but if it's on the other page, then no.
I'll tell you my example. I don't need a visa for European countries, but still got one, a D-type long-stay visa, and when I went to the country, they stamped the other page (6th), not the visa (5th), or the page in front of the visa (4th), so this person tells me that by that time my visa wasn't in use. Then after one and a half months, I left the country and one week later I went again. The stamp that I got there was in front of the visa page (4th). So the guy tells me, that's when my visa got 'activated'. I wonder if it's true or not?