Here in Sofia (Bulgaria) when using the tram/metro/bus you not only need to buy a ticket for yourself but also one for each piece of (large) luggage you carry with you. 

I read about this in a guide book but it is not well advertised on the vehicles, considering I don't speak Bulgarian and have not fully mastered the Cyrillic alphabet. 

On the metro in Budapest the same rule exists, but at least there are signs in English and they also tell you how big your bags are suppose to be to fall under this rule.

You may think that as a foreigner you could try to avoid a fee when a inspector caught you and pretend you did not know, but I've heard of cases where this did not work.

So when should I buy the extra ticket?