I assume you mean something that's considered bad manners. Well, except for the traditional ones connected with bad digestion it would be : pointing your finger at someone. Coming too close. Knocking with your finger on your forehead, you are telling someone he/she's crazy. Call Slovaks Czechs and talk about Prague like it's the capital city of Slovakia. Mistake Slovakia for Slovenia and the other way round. Talk about war like it happened in the 90s (that was Yugoslavia and Slovenia).
Consider Hostel an authentic movie about Slovak life. While Slovak customer service may be considered lacking in quality, chopping hotel guests and torturing them is not really the done thing.
Ask if Slovaks know washing machines, dishwashers, jeans, Coke or Pepsi.