A fraction of ribosomes engaged in translation will fail to terminate when reaching a stop codon, yielding nascent proteins inappropriately extended on their C-termini. Although such extended proteins can interfere with normal cellular processes, known mechanisms of translational surveillance are insufficient to protect cells from potential dominant consequences. Using C. elegans, we demonstrate a consistent ability of cells to block accumulation of C-terminal extended proteins that result from failure to terminate at stop codons. These repressive effects are mediated through decreased protein accumulation without a detectable effect on mRNA levels. 3’UTR-encoded peptides are sufficient to confer the observed effects, suggesting a co- or post-translational mechanism of action. We suggest 3’UTRs may be optimized for sequences that destabilize the attached protein, providing a surveillance mechanism for unwelcome/inadmissible and varied translation errors.