  We have used sensitive LOw Frequency ARray (LOFAR) observations of PSR
B0809+74 at 15--62 MHz to study the anomalously intensive pulses, first
reported by Ulyanov et al. (2006) at 18--30MHz. Similarly to Ulyanov et al., we
found that the spectra of strong pulses consist of distinct bright patches.
Moreover, these spectral patches were spotted to drift upwards in frequency
over the course of several pulse sequences. We established that this drift is
not pulsar-intrinsic, but is caused by the broadband ~20 second-long
enhancements of recorded signal, which influenced the dispersed tracks of
several pulses at once. We speculate on the cause of such enhancements (i.e.
propagation or telescope-related) and the ramifications they bring to the
single-pulse studies at the very low radio frequencies. Depending on the
origin, the phenomenon may also affect the analysis of highly dispersed single
pulses at higher radio frequencies, e.g. Fast Radio Bursts.
