  It is demonstrated that the inherent structure of Boris pusher may result in
the accumulation of errors in numerical integration when estimating the works
of electric fields on particles. This, in turn, leads to an incorrect
estimations of particle acceleration mechanisms. A method of error correction,
reducing error to the acceptable levels of $<10\%$, is implemented. Numerical
integration error in question is most pronounced in scenarios where substantial
number of electrons are injected into the accelerating structure through wave
breaking of plasma waves i.e. in near-critical density plasma.
