Although functional RNA is generally protected against degradation, defects or irregularity during RNA biogenesis lead to rapid degradation. Cellular surveillance mechanisms therefore need to distinguish aberrant, erroneous, damaged or aging transcripts from normal RNAs in order to maintain fidelity and control of gene expression. The detection of defects seems to be primarily based on functionality or aberrant rates of a given step in RNA biogenesis, allowing efficient detection of many different errors without recognition of their specific nature. We propose that the addition of non-templated nucleotides to the 3' end of mRNAs and small non-coding RNAs, 3' tagging, is the primary means by which malfunctioning RNAs are labelled, promoting their functional repression and degradation. However, the addition of non-templated nucleotides to transcripts can have diverse effects which vary with location, length, substrate and sequence.