  We revisit here some aspects of the interpretation of dynamic force
spectroscopy experiments. The standard theory predicts a typical unbinding
force $f^*$ linearly proportional to the logarithm of the loading rate $r$ when
a single energetical barrier controls the unbinding process; for a more complex
situation of $N$ barriers, it predicts at most $N$ linear segments for the
$f^*$ vs. $\log(r)$ curve, each segment characterizing a different barrier. We
here extend this existing picture using a refined approximation, we provide a
more general analytical formula, and show that in principle up to $N(N+1)/2$
segments can show up experimentally. As a consequence the interpretation of
data can be ambiguous, for the characteristics and even the number of barriers.
A further possible outcome of a multiple-barrier landscape is a bimodal or
multimodal distribution of the unbinding force at a given loading rate, a
feature recently observed experimentally.
