Thought just explain that there is nothing much to fear about this error. This error usuaully occurs when some process tries to create a temp segment in a tablespace be it temporary or permanent. WE have to understand that temp segment does not always mean that there is a sort operation involved. During DDL commands also such temp segments are required and hence if such an operation is big, for a particular tablesapce such error occurs. This keeps happening when new objects are created like indexes, table, constraints etc.
Hence see if any user was trying to create a object. The best solution is to coalesce the tablespace and most of the cases the problem will get solved. Else if the operation which failed is essential then add space to the tablespace.
This also happens during index rebuild, table move etc.
This is not a very severe error.