No matter how well your cat is walking on a leash never let your guard down, especially if your cat has been a strictly indoors cat. There will be sounds that are new to them. There will be experiences they've never had to deal with before.
Doesn’t matter how laid back your cat was inside the house, don't assume that she will take the great outdoors in stride. Be sure that your cat is wearing the most secure harness you can find and afford and that you are always holding the leash securely. The last thing you need when your cat is panicked is to have the harness come off, or the leash slip out of your hand, and your cat running madly and blindly out into the street.
These are three experiences I've had with my cat outdoors.
1. New (to them) Sounds
Until my cat went outside I had no idea that she would be panic struck at the sound of the ice cream truck - not the van itself - but the music that it played. At first I didn't know what it was that was terrifying her. Only when the music got louder and she became more frenzied did I realize. Even when she is just in the backyard she can hear the jingle from a couple of blocks away and will immediately start running, trying to find a place to hide.
After that first experience of trying to hold a frightened, struggling cat and getting thoroughly scratched, we learned not to go walking when its ice cream truck time.
The truck comes around about the same time every day so we can avoid this problem, but there are the out of the ordinary sounds.
2. Unusual Sounds
One day my cat was in the backyard on her long leash, enjoying the sunshine, when one of those big advertising balloons came overhead. Not for the first time, but this time it let out air or gas or whatever they use that makes a loud whooshing sound. My cat froze in place, her eyes widened and bulged and she peed on the spot. When it made the sound again, she started to run, making a horrible wailing sound, peeing the whole time.
Luckily, being in the yard she could run into the house. If we had been out on the street I would have been scratched, bitten, and peed on. And there is really no way to predict when or if this is going to happen.
3. Dogs Running Loose
My cat lives with dogs. Meeting a dog on the street that's on a leash doesn't bother her at all. She will either ignore it or sometimes (rarely) go up to say hello.
But an unleashed dog, galloping up to her - even if he's friendly - has her back hairs up, hissing, spitting, with claws fully extended. The only thing I can do is try to keep the dog from getting close enough for a fight (not easy), and hope that its owner is close by and will call it away.
Twice in the past year we have been confronted with this situation. The first time the dog decided it had better things to do than tangle with a cat and left on its own. The second time the owners realized their gate was open, the dog was gone, and they came out to call him home.
My cat has been going for walks for many years now; loves her walks; demands to have a walk everyday. But experience has taught me to keep a close eye on what happening around us, and to always be monitoring her reactions. Never assume you know how your cat will react in new situations. They always manage to surprise you.
