Are there consequences to stamps in my passport that are not from border control?

Finland is notorious for systematic inspections of passports of visa applicants with the purpose of finding such stamps.

The passport above is one received back by a visa applicant with the note that says: Respected sir/madam, hereby we communicate it to you that your passport is not a valid travel document because of the following defect found in it: unofficial stamp on p. 9. Your visa application will be with the visa section of the Consulate General in the status of "temporarily suspended". If until 30/9 you bring a rectified/new passport, a visa will be issued for the consular fee you already paid.

Moreover, they even apply this policy to memorial stamps that border guards in UAE place to travelers' passport, despite the fact that travelers have no option to decline them.

Of course, if you don't need a visa, Finnish border guards won't care, which comes in accordance to the principle of fairness and equal treatment of all individuals. Such stamps are also issued in Finland itself.

Border /visa / status stamps in your passport are of interest to the nation you are attempting to enter. With rare exceptions (Israel vs. some Arab countries) they are of zero interest or legal status elsewhere. Unless you are attempting to claim that a stamp is legitimate for the purposes of employment or immigration fraud it's unlikely that they are going to be a problem.

Don't go overboard, don't overwrite currently-valid official stamps, and definitely avoid politically-sensitive areas and it's unlikely anyone will care. They have bigger fish to fry.

Don't get these. My friend was almost denied boarding an international flight because of a fake Machu Picchu passport stamp. Taiwan has rejected several people from entry for fake stamps as well.

If I remember correctly, the souvenir shop at Dorasan Station, which is the last stop in South Korea of the erstwhile Seoul-Pyongyang line, has stamps which look like North Korean entry stamps. Such a stamp could cause problems when entering other countries, and I believe they warn tourists about that.

I also got the (in)famous Macchu Picchu stamp (no idea why they seem to so nonchalently want every tourist to stamp their passport), and I almost got refused departure by the Chinese border control when crossing from China into Nepal, since they couldn't recognize the stamp and insisted that the passport was damaged with unofficial markings. We spent more than an hour there and finally they let me pass after calling and discussing with a superior. So yeah you'd definitely want to be safe and I would say it's not worth it to risk your passport which is the single most important thing when traveling internationally. Use a separate booklet to collect the stamps if you want to.

Andorra does stamps too but that's a proper country as is Monaco. Most border officials don't look at the stampage. I've got a lot of stamps and only 4 times have border guards taken an interest,

1. Walking over the Montenegro/Croatian border a Croatian border guard looked but it was only out of interest in all the palaces I'd been,

2. At Beirut but looking for Israeli stamps (in old passport!),

3. Israel!

4. Harwich to Hoek van Holland ferry in Harwich (England to Netherlands) again think he was just interested in the places I'd been.

Got one in Ushuaia at the tourist booth showing a map of the island because it was personally meaningful, know it is at best semi-official. (The Port Lockroy, Antarctica, stamp next to it is slightly more genuine.) Never had a bit of trouble with it but then I have a fair number of stamps and it is in dark ink so it doesn't stamp out. Got tourist office ones from Monaco and on request at the Andorra border, as well as on request in some Caribbean ports, and the Barbados one says "immigration-souvenir"--again, no issues whatsoever. I've heard that Finland can give problems with clearly unofficial ones, but I suspect it comes down to whoever sees them and how they feel. Many years ago I was traveling by bus to and through Nicaragua and Honduras. Read that passport stamps from Communist countries made entry invalid to these nations, and I had plenty from Eastern Europe. I figured roadside border people wouldn't notice, and indeed they did not. Best advice I can give is to not over do it, and stay away from bright weird ones.

I have a few souvenir passport stamps from the Caribbean when on a cruise. It was never a problem with officials. In Lebanon my passport was looked upon for Israeli Stamps at the Beirut Airport. One other time my passport upon arrival at Heydit Intl. Airport in Baku Azerbaijan the official saw an Armenian Visa (used) in my passport. She went through all my pages then a higher official looked at all my pages. I was never told yet knew they were looking for passport stamps/visa of Nagorno Karbakh which is Armenian occupied Azerbaijan. I had an Azeri Visa that wasn't the problem so they stamped me then let me go.