As long as you have a valid ticket (Eurail pass, correctly set-up) and compulsory reservation if required for that train then you won’t have anything to worry about.

See this page for how to set up and use the pass.

https://www.interrail.eu/en/interrail-passes/interrail-mobile-pass/getting-started

Where are you hearing about fines?

Obviously, if you’re travelling without a ticket (and using a pass incorrectly - eg not activating a travel day - might be considered to be travelling without a ticket) then you risk a fine. It will be down to the policies of the individual railway company you’re traveling with (nothing to do with Eurail) and the judgement of the ticket inspector as to how to handle it.

General advice would be to make sure your pass is valid for the train company, make sure it’s in date, and a travel day activated today. Make sure you have this train listed in your diary, and a QR code to show (if you’re using a mobile pass) 

If you have any problems or unsure about something, approach the ticket inspector first, before they get to you. They’re not monsters and they usually good at telling the difference between a fare-dodger and somebody with a genuine issue.

My Daughter just had a huge fine as the phone she had the App on was lost and she got backed into a corner and her and her Partner got charged nearly 500 Euros:-(

i am Mad as Hell!

this was in the Train to Barcelona today.

You can't travel without a ticket, which is what she did. She should move her pass to another device, which is what a short Google search would have shown:

https://www.eurail.com/en/eurail-passes/eurail-mobile-pass/mobile-pass-faq/what-should-i-do-if-i-lose-my-phone

She had a Paper ticket?

Looks like a mobile pass: "the phone she had the App on was lost”

Plenty of good advice but one quick point - some posts in the past have referred to taking a screenshot as a back-up if the app causes a problem. Whilst this may work please remember that that is technically not a valid ticket. It is easy for the inspector to see it is a screenshot as it will not scroll to show both the top info and the lower info. This is often an indicator of possible fraud - anybody can send a screenshot to a “friend”.

It is cases like this (or not having a mandatory reservation or not having the current train in the listing) that cause most problems.

However please remember how many successful Interrail journeys are completed every day.