When in Rome, DON'T EAT DOG FOOD ... or you'll regret it, just like Serena Williams -- who just went nuclear all over her hotel toilet.
Serena's in Italy for a tennis tourney -- and became curious when she saw her hotel had a special pet menu that included a salmon and rice dish.
Serena ordered it up for her dog Chip -- but decided to taste a spoonful for herself first.
"Let's fast forward to two hours ... I just ran to the toilet like i thought i was gonna pass out!"