except you'll be on occupied territory and will be breaking Ukrainian law, you might get a prison sentence if you ever go to Ukraine again
except you'll be on occupied territory and will be breaking Ukrainian law, you might get a prison sentence if you ever go to Ukraine again
Then the obvious question to "where you exactly you have been there and what for" will come out unexpectedly for you and you will start making up a bullshit story on the spot (after being on the road for the last whole day) and spit out enough signals to look into you even more.
At this point you are taken to a room for a talk and asked to show pictures in your phone, hotel confirmations, train tickets, etc, which obviously show you have been where you are not supposed to and you lying as well. If you panic and refuse to show the phone contents, you are likely just barred from entrance on the spot and put on a list. If you do show the phone contents and there isn't anything noteworthy, but your story is not coherent enough (or the opposite: coherent enough to the point you prepared for it), you get at date with a three letter agency who is supposed to not rely on feelings.
The "how do they know" is always a bad idea as long as people need to know and your actions leave extensive digital papertrail.
Lots of things in immigration require you to honestly self-report and have no mechanism to validate at the point-of-entry. For instance, many countries without visa requirements will still ask about criminal records in your native country, but have no access to the data to verify if you tell them you've never even had a parking ticket.
But if you get caught later down the line, then they are going to use your lies to make life very hard for you.
What I was mentioning though is that countries will let you have a second passport from a single country if you have issues like this, to avoid showing the stamps in one passport.
But legallity, politics and bad taste aside, it just seems a dangerous place to visit. It wasn't long ago that an Ukrainian missile was shot down over Crimea which ended falling down on a beach in Sevastopol and killing 4 people.
(Please correct me if I'm wrong.)