“Thought to result from splicing errors, circular RNA species possess no known function.” Source RNA Journal 2013.
RNA usually occurs in strands, but can also form cricular loops. It's predominantly found in the cell's cytoplasm - i.e. outside the cell nucleus.
It's now known that Circular RNA (circRNA) can code for proteins, but the function is still, in general, unknown.
Many geneticists now tend to regard circRNA as functional rather than accidental. Several theories have been out forward to explain possible functions - including as a 'sponge' to 'soak up' genetic errors during cell divisions.