It's very hard to resist petting every dog you see, I know, but most people know not to interrupt a service dog. Service dogs are usually easy to distinguish because they're usually wearing a vest or harness. They probably get lots of pets at home, but they shouldn't be distracted on the street while they're looking out for their human.
But that's about the extent of what most people know about service dogs. However, a recent PSA is circulating to inform people that these dogs have all sorts of training to make them useful to humans with medical issues.
Twitter user Melissa Hope, or @lissalet, recently posted a story about someone who tripped and fell. Their dog had been trained to help them in the event of an epileptic seizure, and that's how the pup interpreted the fall. It ran off to find another person to come help, but the person it met was not very helpful at all.