  We report a serendipitous detection of rapid, large amplitude flux density
variations in the highly core-dominated, flat-spectrum radio quasar 1156+295
during an observing session at the Very Long Baseline Array (VLBA). The source
was observed as a part of the MOJAVE survey programme with the VLBA at 15 GHz
on February 5, 2007. Large amplitude variability in the correlated flux
density, unexplainable in terms of the source structure, was first discovered
while processing the data, and later confirmed by calibrating the antenna gains
using 24 other sources observed in the experiment. The source shows variations
in the correlated flux density as high as 40% on a timescale of only 2.7 hours.
This places 1156+295 between the classical IDV sources and the so-called
intra-hour variables. The observed variability timescale and the modulation
index of 13% are consistent with interstellar scintillation by a nearby, highly
turbulent scattering screen. The large modulation index at 15 GHz implies a
scattering measure that is atypically high for a high galactic latitude source
such as 1156+295.
