  HHL quantum algorithm to solve linear systems is one of the most important
subroutines in many quantum machine learning algorithms. In this work, we
present and analyze several other caveats in HHL algorithm, which have been
ignored in the past. Their influences on the efficiency, accuracy and
practicability of HHL algorithm and several related quantum machine learning
algorithms will be discussed. We also found that these caveats affect HHL
algorithm much deeper than the already noticed caveats. In order to obtain more
practical quantum machine learning algorithms with less assumptions based on
HHL algorithm, we should pay more attention to these caveats.
