Stray light (flare) is formed inside cameras by internal reflections between optical elements. We point out a flare effect of significant magnitude and implication to snapshot hyperspectral imagers. Recent technologies enable placing interference-based filters on individual pixels in imaging sensors. These filters have narrow transmission bands around custom wavelengths and high transmission efficiency. Cameras using arrays of such filters are compact, robust and fast. However, as opposed to traditional broad-band filters, which often absorb unwanted light, narrow band-pass interference filters reflect non-transmitted light. This is a source of very significant flare which biases hyperspectral measurements. The bias in any pixel depends on spectral content in other pixels. We present a theoretical image formation model for this effect, and quantify it through simulations and experiments. In addition, we test deflaring of signals affected by such flare.