Zipcar's Black Friday car-sharing outage has left some users stranded in the cold for over four hours.

You may have seen bike-share schemes in some cities - you know, those that you can unlock, ride and then leave in another location - but have you seen vehicle schemes? Maybe I'm a little too small-town, but I didn't know. Not until I heard that they were causing all sorts of kerfuffle in the last week.

According to 404 Media one of these car share schemes, Zipcar, experienced a outage which caused a "clusterf ***" during Black Friday. Customers were locked out of their rented cars, sometimes for long periods, and then charged summarily for the privilege. Zipcar's app was down.

Imagine this: You rent a vehicle to "zip" into the store to buy some bread. The next thing you know, you're locked out, waiting on the curb to be told that you owe cash for no reason. You will be refunded eventually, but it may take some time.

We don't need hypotheticals. Here's what 404 Media claims one customer said:

"This is insane. I rented a car, went to the store to get a quick drink and then the car was locked. I've waited over 4 hours outside in the cold. No help, different answers, and stuck waiting an hour for someone to speak to you and no help. My house keys, my house contents and all my belongings are inside. This is so frustrating and crazy."

This is all because of a Zipcar app outage. The app allows you to lock/unlock your car, as well as start/end your rental. If the app stops working, then you're screwed. Ars Technica notes that this wasn't always the case. Zipcar used physical keys to unlock cars, and members received Zipcards. It's been a while since that was the case. Now, it's all done through an app.

Even though I'm averse to risk, I can't deny that it's not ideal to have an app act as a single link in a chain of car-sharing services. If isa link that holds the chain together, then you better make sure that it's a good one.

It's possible that the SMS/MMS network is to blame for the failure of the Friday service.

Zipcar told Ars Technica that during part of Friday afternoon we experienced a rare site outage due to increased traffic. Unfortunately, the Black Friday promotion has caused SMS delivery service restrictions on our site and other sites.

"For a small number of our members, who were not already signed in to our mobile app, it resulted into login difficulties that affected their reservations." We are working to resolve this issue and prevent it from happening again.

The company told Ars, "responses vary by case, but include refunding of reservations, giving driving credit for future trips and refunding alternative transportation."

Zipcar did not reveal how many people were affected but it did state that "a small number of our members were affected who had not yet logged in to our mobile app".

I don't like to store unimportant files in the cloud, for fear of something going wrong. I'm not the target audience, but I know that I won't risk a long journey with someone who relies on an app to keep me safe. But that's just me. At least, people are getting refunded.