Biological wastewater treatment plants operate under intrinsically unsteady influent conditions, and sudden increases in organic loading can destabilize carbon removal, nitrification, sludge structure and membrane filtration. This focused critical review synthesizes three complementary investigations covering a two-stage activated sludge-biofilm reactor, nitrifying sludge exposed to variable organic loading rates, and a membrane bioreactor subjected to repeated shock events. The reviewed evidence shows that apparent recovery of chemical oxygen demand removal can occur considerably earlier than recovery of nitrogen conversion, making organic removal alone an insufficient indicator of biological resilience. In the hybrid reactor, short-term influent chemical oxygen demand shocks of approximately two to four times the normal concentration produced progressively higher effluent peaks and recovery periods of about 7-27 h, while severe shocks suppressed nitrification and shifted the dominant cultured bacterial morphology. In nitrifying sludge, increasing organic loading modified loosely bound extracellular polymeric substances, reduced protein content and the protein-to-polysaccharide ratio, deteriorated aggregation and separation properties, and caused system collapse at 0.75 kg COD kg−1 MLVSS d−1 with incomplete recovery after load reduction. In the membrane bioreactor, higher shock magnitude accelerated residual fouling and transmembrane-pressure jumps; increasing filament abundance indicated that clogging and fouling acted synergistically. Across configurations, dissolved-oxygen competition, heterotrophic overgrowth, extracellular polymeric substance composition, biofilm retention and filamentous growth emerge as the principal mechanisms governing resistance and recovery. The review concludes that robust design requires multi-parameter monitoring, protection of nitrifying populations, equalization of peak loads and reactor-specific control of solids separation. The limited three-study evidence base also reveals a need for standardized shock-loading protocols, full-scale validation and integrated microbial, hydraulic and filtration diagnostics.