Anyway it's doesn't seem to be related to using any extension at all (as you can see, I tried with a different one).
Try this:

Start with a clean history.
Go to http://html5test.com or http://acid3.acidtests.org.
Go to https://www.w3.org, http://www.w3schools.com or https://www.javascript.com.
Clear one by one all lines from the history.
Start typing in the address bar. Only sites from step 2 are suggested.

No it isn't, I remember 5 days ago clicking on a random website for direct downloads that redirected me to the actual download site and there same bug affected the browser.

Try this:

• Start with a clean history.
• Go to http://html5test.com or http://acid3.acidtests.org.
• Go to https://www.w3.org, http://www.w3schools.com or https://www.javascript.com.
• Clear one by one all lines from the history.
• Start typing in the address bar. Only sites from step 2 are suggested.

I used the Adblock Plus addon.

OK, just did a little test here and I guess I could see what you mean.

I did some testing on a new instance of a portable Opera, so I don't screw my Opera instance.
I installed AdBlocker Ultimate but after uninstalling it no page opened, only after installing. Anyway, after removing it, I noticed what you said. Having deleted those entries one by one, as well as the few pages I had visited, Opera kept suggesting the ad blocker site, its /contribute page, addons.opera.com and also these forums. Some other sites I visited did not appear, so I'm not sure about the logic behind this. At first I thought that sites opened from within an Opera site would remain, but then some other site I manually entered also remained... Strange behavior.
If I clear last hour's history, they are gone for good.
So basically yes, there's something fishy there, like if there are some history entries that are not being listed/deleted.

Yes, personally I don't mind having the official Opera addons page displayed as a suggestion, and that is why i didn't mention it at first, it's the fact that users get 6 lines of suggested sites and for people that keep track of what they've been doing for months in their browser, finding 3 ghost items stuck to your address bar defeats the whole purpose of having suggestions in the first place which is being able to go back to the stuff you really cared about.

Step 1: Install ad-blocker.

What ad-blocker?

I used the Adblock Plus addon.

I installed AdBlocker Ultimate but after uninstalling it no page opened, only after installing. Anyway, after removing it, I noticed what you said. Having deleted those entries one by one, as well as the few pages I had visited, Opera kept suggesting the ad blocker site, its /contribute page, addons.opera.com and also these forums. Some other sites I visited did not appear, so I'm not sure about the logic behind this. At first I thought that sites opened from within an Opera site would remain, but then some other site I manually entered also remained... Strange behavior.
If I clear last hour's history, they are gone for good.

So basically yes, there's something fishy there, like if there are some history entries that are not being listed/deleted.

Step 1: Install ad-blocker.

What ad-blocker?

Now the thing I've found to be a small bug is that sites that create extra and annoying tabs such as ad-blocker and a bunch others, once you've uninstalled its addon for example, a history entry will be created and in cases like this, even if you tick on the history's x mark , this "visited" site shows in the address bar as a suggestion

Give it a try with the triggers I stated in the original post and let me know what happens

Step 1: Install ad-blocker.

Step 2: Uninstall ad-blocker.

Step 3: Go to History and delete the redirected page opened by adblocker upon uninstallation.

Step 4: Check the deleted history entry showing as a suggestion in the address bar.

When that site appears, what icon is at its left? That icon indicates where that site is coming from: history, bookmarks, speed dial, etc.

Yes, it is the history's icon

Usually, when you type the first letter in the address bar you get a heart icon if it's a bookmark, a google favicon suggesting an engine search etc, or just the clock icon I get that one clock that's why I'm sure that it is a history record

Look I'm playing around with the browser for this month, getting used to it and giving it a shot, so it's not a big deal for me right now and the solution would be to delete your browsing history for the past hour if you noticed this early enough. My take on it is that, if you didn't find out about this address sticking to your browser before a day has passed, you'll be forced to either delete all of your history of days, or simply get used to seeing those suggestions in spite of being manually deleted already.