I recently rescued a young(15mth) Rottie.
I tried him out with my dogs and he is the perfect gentleman.
I then thought, after a week or two, that I would ask a few friends if he could 'come play' with thier dogs. Again no problem.
Unfortunately while at a friends with a Leonberger, I asked if she was in season, they were left alone.
The next day my friend called and told me she thought the bitch was now in season. I went straight up and sure enough Kiera was in season.
We waited a few weeeks and luckily for us the bitch is definitely not pregnant. We've had her scanned. It's now 10 weeeks since the event so we are certain.
If it had resulted in pups, even though both dogs are of excellent temperament, it would have been a strange mixture.
In future I'm not only going to ask an owner if a bitch is in season I'm going to check for myself. The last thing I want is my dog to be fathering pups.
I haven't had him neutered as yet because my other two male dogs were fine before they were neutered but started to fight when they had the op. They have settled now but it's taken 2 years of hell and when it's a bullmastiff and  JRT the last thing you want is a fight.

LMAO @ rottenburger :D

I'm so pleased they hadn't mated but if the bitch had been pregnant and her owner didn't give her the jab then I would have helped with the pups.