At least in Germany that's the case.

Every contract is legally binding in Germany, even verbal contracts or in this case price tags (to some degree). Obviously other laws may invalidate them and verbally is hard to prove. For example if you advertise onetime off prices for a week to lure people in the store you have to have a reasonable amount of these items to be available through the week, otherwise people are eligible to get the offer or compensation.

Adding your own sticker would probably be fraud and easy to prove for the store (not matching sticker, no plans to reduce prices, ...).

Otherwise anyone can just run around with a 10/20/50% off sticker and force any store to sell them whatever they want for much cheaper.

Or they can just steal it, it's just as legal. In my experience this is law in a lot of the EU, including Germany and a bunch of Eastern European places.

In my case, it wasn't a misplaced 90% off sticker, it was just that the normal price tag on the shelf was printed with one zero less. It was also a "premium" item at the time, so the price wasn't that much off, just cheap. It wasn't just a bunch of shrimp, it was ready made, cleaned, arranged into a neat circle with dipping sauces in the middle.

On the other hand, I had a thing where Microsoft was introducing Skype to a country where the local currency was around 200:1 to the dollar. They messed up the currency conversion, and it defaulted back to 1:1, giving everyone a 99.5% discount on consumer electronics. It was obviously not honoured, and the law was clear, so no lawsuits either.

In most of US, the price tag is a legally binding offer, and its presence is required by law in most cases. Here for example is NYC law:

New York City Administrative Code
Title 20: Consumer and Worker Protection
Chapter 5: Unfair Trade Practices
Subchapter 2: Truth-in-Pricing Law
§20-708 Display of total selling price by tag or sign.

All consumer commodities, sold, exposed for sale or offered for sale at retail except those items subject to section 20-708.1 of this code, shall have conspicuously displayed, at the point of exposure or offering for sale, the total selling price exclusive of tax by means of (a) a stamp, tag or label attached to the item or (b) by a sign at the point of display which indicates the item to which the price refers, provided that this information is plainly visible at the point of display for sale of the items so indicated. This section shall not apply to consumer commodities displayed in the window of the seller.

§ 20-708.1 Item pricing.

e. Price accuracy. No retail store shall charge a retail price for any stock keeping item, whether or not exempt under subdivision c of this section, which exceeds the lower of any item, shelf, sale or advertised price of such stock keeping item.

City inspectors may perform random checks to compare tag price to scanner price at checkout and fine store $25-$100 for every incorrect/missing tag, and may repeat the inspections every 24 hours until problem is solved.

If you run around slapping your own discount stickers it wouldn't count since the store didn't do it, you are just committing fraud. The store would be on the hook if it continued to display the fraudulently-mislabeled product for sale after being made aware of it.