  Ionized nebulae are key to understanding the chemical composition and
evolution of the Universe. Among these nebulae, H~{\sc ii} regions and
planetary nebulae are particularly important as they provide insights into the
present and past chemical composition of the interstellar medium, along with
the nucleosynthetic processes involved in the chemical evolution of the gas.
However, the heavy-element abundances derived from collisional excited lines
(CELs) and recombination lines (RLs) do not align. This longstanding
abundance-discrepancy problem calls into question our absolute abundance
determinations. Which of the lines (if any) provides the correct heavy-element
abundances? Recently, it has been shown that there are temperature
inhomogeneities concentrated within the highly ionized gas of the H~{\sc ii}
regions, causing the reported discrepancy. However, planetary nebulae do not
exhibit the same trends as the H~{\sc ii} regions, suggesting a different
origin for the abundance discrepancy. In this proceedings, we briefly discuss
the state-of-the-art of the abundance discrepancy problem in both H~{\sc ii}
regions and planetary nebulae.
