There are two different things going here. First is what the law says, which in simple terms is that anything literary, musical or artistic which someone creates, can qualify for copyright if it is original and the person who made it qualifies in terms of their nationality or residence for protection under the local laws. The latter can be largely set aside because due to several international treaties, reciprocal arrangements mean that citizens of virtually all countries are entitled to have their copyright works protected throughout the world.
However, there are some specific exceptions to the general (legal) rule that you shouldn't copy other people's copyright works without permission, which I will cover in a moment.
And because these legal rules fall within what is known as civil law, it is up to the author or creator of the work to take action if they think their work has been infringed. Not everyone does, and in many circumstances they actually want others to share their works. That applies in particular on the internet where users of social media sites wish to interact freely with others, and aren't concerned about exploiting their work commercially. Add to this websites who facilitate such social interaction (which is for commercial gain), and whose terms and conditions often require copyright owners to give up some of their rights in exchange for using the site, and you have a very different environment from the strictly legal one.
On that basis it is very difficult to give you simple advice about sharing and re-using other people's work which you encounter on the internet. In general, I would say that you should always seek permission even if only out of politeness, unless you are very sure that the author means for their work to be freely copied. Usually, the more knowledgeable among them will put a so-called open licence (such as the best known one, Creative Commons
) on their work to avoid misunderstandings. Where you find such licences you should abide by the various stipulations, such as not using the work commercially, or the need to credit the original author and so on.
Finding the original author of something which has been re-tweeted or shared or copied in a similar fashion can often be very difficult. but that doesn't mean that you can ignore it completely. However if something becomes a meme or is so viral that pinning down the source is virtually impossible, it may be because the original author prefers the fact that their work has gained widespread popularity and is not concerned about protecting copyright.
The danger comes when you find something you would like to share, and you think that the author would be happy for you to do this without getting permission, but it turns out that they have not actually created the work from their own imagination or creativity but have taken another copyright work and adapted it. A classic example of this sort of genre is music sampling and mashups. The courts are constantly dealing with cases where Band A has used part of a song by Band B in this way, and Band B is seeking hundreds of thousands or millions of dollars in damages. That is why you need to be careful about taking things at face value.
As for what may happen if you do infringe someone else's copyright, there are several scenarios which could apply. If the owner is some large corporation like Disney or one of the record labels, then you can expect some fairly heavyweight legal pressure right up to being sued and having to pay damages. However, in many cases where the rights owner is an individual, you may just receive a takedown notice, which if you comply with it, will probably be the end of the matter. The more tricky situation arises where the rightowner makes his or her living from the work they make (say a photographer or artist). They will almost certainly want some recompense for their work having been copied without permission, but they may not have the time or resources to actually take you to court. If they demand a reasonable fee then it may be safer and easier to just pay up, but if the fee is clearly unreasonable, then you may need to stand your ground and negotiate a lower fee.
And finally you ask: "Is there any such case for my knowledge?" I am assume by that you mean is ignorance a defence when it comes to copyright infringement. The answer is no. Even though you may have been reasonably sure that you were OK to copy something, if the true owner of the copyright alleges you have infringed then your only recourse is to one of the various legal exceptions which apply. I am assuming that you are asking from the point of view of a person living in the UK; if not then the law where you live will be different in this respect and you should check locally.
UK copyright law allows part of a work to be quoted for general illustrative purposes, or for the purposes of private research or study, or for the purposes of criticism or review (say, something like the review of a book or play), or for the purposes of news reporting, and lastly for the purposes of parody or caricature. Each one these categories has itsr own special rules and stipulations which need to been followed. For instance when quoting or copying something for the purposes of reporting current affairs, you may not use a photograph, or if you are quoting something for the purposes of criticism or review, the work must have previously been made available to the public before you can do this. You can read the complete set of exceptions here
, but be aware that some recent changes have yet to be incorporated into the main statute and you need to also check the pink box at the top of each section where it says 'view outstanding changes'.
It is a sensible move to put a short disclaimer on your web page to the effect that if you have inadvertently infringed someone else's copyright, you will immediately remove their work if informed of the fact. It doesn't have much legal value, but it may placate any copyright owners not bent on extracting money from you.
I hope this answers your question.