First of all - I reported this error using Opera Bug Report Wizard on May 28th while it was happening in Opera Beta. Bug report got ID: DNAWIZ-29482. I didn't get any reply.
You can reproduce this error only if your mouse has extra backward/forward buttons.
Clicking mouse back button goes back by 2 pages instead of 1, example:
I'm on site A then I navigate to site B and then to C. Pressing backward mouse button gets you to site A instead of site B
When i click and hold back button on my mouse then opera goes back by only one site, then I release that button and it goes back one more page. Same thing happens with forward button. I checked that on 2 different computers running Linux Mint with 2 different mouses. Button numbers in xinput test are the same for both computers so I assume that it's error that could possibly affect more users than just me. Strangely enough this behavior does not occur on Windows.
I checked which buttons are clicked exactly on my mouse by terminal command "xinput test <device_id>", it seems that someone changed opera's behavior by triggering forward/backward navigation on both key press and release:
button press 8
button release 8
button press 9
button release 9