I know this isn't the first time we hear about these errors but there are many problems that can occur when booking online:
Session Timed Out / Broken link / No return message from supplier system etc etc
The internet is great but travel systems are often complex and having to connect with various other system. If one of the suppliers is having issues, may result in an error yet something may have actually confirmed.
Now, if you are getting a message like you said with nothing else there such as please contact a memeber of our team or anything then from a customer point of view i can understand why you did what you did but also, if i had inputted my card details, i probably wouldn't have done it again without checking as i have had a problem before but if you are maybe not that familiar with online booking then this needs to be considered too by them.
The company i work for probably books one tenth of what OTB do and even on our system, if one of the elements fails it will say what has happened and that there has been an error and ask the customer to contact the call centre.
With the size of OTB, you would think that they would have this in place, we are not a huge company or anything, only about 15 staff total so if a small little company can do it, then a company the size of OTB who spends thousands on advertising and google etc should maybe reinvest some of this into their booking system. In this day and age, a system should be fool proof.
Thanks