This happened to me the other week and has come back to bite me. I fell over on my way to work a couple of weeks ago 
and got mud up my jeans. No problem as I always have spare clothes in my office for when it's pissing down.
However, on my way in I bumped into a colleague who commented on the mud. I replied and said the dog jumped up at me.
"Oh, I didn't know you have a dog. We must go walking together one time", she said.
Rather than me saying "Oh no, sorry, it was just a random dog" or actually just telling her that I went arse over tit, I went "
Ooh, lovely."
I got to my office, shook my head at what an idiot I am and thought nothing more of it.
This morning she has texted me "Hi Billy. Just wondering if you fancy going over the fields on Saturday with the dogs? About 11am."
I have no idea what "the fields" are but assume I would know if I actually had a dog.
I don't feel like I can text back and say it was all an elaborate bullshit to cover the fact I fell over because this makes me look like a knob on so many levels.
So, what are my options here?! Tell her the dog is dead. No, that's horrible. Tell her I'd love to and then beg, steal, borrow or buy a dog in the next 40 hours. Possibly but that's just adding to the lie. I haven't decided.
Tell me your stories of white lie spiraling?