Sometimes you make an edit, in which information is altered or added to a page, when the previous information was referenced. Now, that's all right; not all references are accurate, new information can become available, and so on. However, in many cases, the altered or added information will be altered or added in such a manner that the citation referencing the previous information, appears to be referencing the new information as well. Altering numbers, as in census results, climate data, or production total tables, is a common example of this "reference hijacking"; less common, but still happening often enough to be noticed, is slipping in additional prose text inside a sentence that is followed by the citation tag. These are often good-faith errors by newbies, who simply overlook the existence, or don't know the purpose, of the <ref> tags. While it can be passed as a mistake, this trick is, however, also a favourite tactic of vandals.

Bottom line: don't hijack references. When changing referenced content, add a new reference, or remove the original reference if it's erroneous and replace it with a {{citation needed}} tag, preferably an explained one {{citation needed|reason=your explanation of the issue}}, and with an edit summary repeating the same explanation of the issue. Don't change the numbers in the table and leave the original reference for the entire table; do the same as you would in the first case mentioned.

When adding wholly new information, make sure you are not adding it just before an existing <ref> tag. Adding new information in the middle of a paragraph that used a single citation at the end is admittedly tricky, and a good argument for "don't reference a whole paragraph to one cite at the end". But still, if you're adding something: cite it! The usual solution to adding new information to a paragraph that had only one source is to move the citation to before your insertion, add your new material with a new citation, and add a short repeat citation to the original source at the end of the paragraph. Another approach is to use repeat citations for each sentence in the paragraph other than your addition with its own source.

• WP:Citing sources § Text–source integrity – the Wikipedia guideline on the matter
• WP:Citation underkill and WP:Why most sentences should be cited – essays suggesting to cite every sentence
• WP:Cuckoo editing