So you sign up on website.com, you click on the notorious I accept tos and privacy policy checkbox (many without reading it) and you pay for the service.
Month later, website changes their TOS page adding few new paragraphs without notifying you that allow them to bill you $10 every month.
When you complain, they say: Read our TOS, you agreed.
How can you prove and show the original TOS changed? That you never agreed to this version of tos at all?
You can use internet archival services like the internet Wayback Machine.
Check out this question that basically discusses the same topic from the other side – the website owner. Judging from WBT's answer over there it may depend on the judge whom he/she will believe when this case is brought to court and if neither parties have good evidence.
Ideally of course you saved a copy. But also the website owner would need to be able to show a copy, the exact revision of it when you signed up to be precise. But since the website owner obviously doesn't obey to the law who knows what will happen...
You can take a screen shot of the web page of the terms you agreed to and post that screen shot to a third party image hosting web site like Flickr, Tumblr or Imgur. The purpose of the third party host is to provide an objective and reliable timestamp for the screen shot.
Alternatively (but perhaps less compelling) you could copy the text of the TOS and save it to a file again on a third party hosted service with a timestamp. Like Google Docs.
