RNA binding to high-density oligonucleotide arrays has shown tantalizing differences with solution experiments. We analyze here its sequence specificity, fitting binding affinities to sequence composition in large datasets. Our results suggest that the fluorescent labels interfere with binding, causing a catch-22. To be detected, the RNA must both glow and bind: without labels it cannot be seen even if bound, while with too many it will not bind. A simple model for the binding of labeled oligonucleotides sheds light on the interplay between binding energies and labeling probability.