When measuring signals in electronic circuits some times what you see on the screen is not what you have in the circuit. Parasitic couplings, mismatching, noise pickup, and many other anomalous effects can give you the wrong picture of your problem.
When designing EMI filters how and where the filter is installed is critical to obtain the desired insertion losses. Usually, the effect of a wrong location or installation is not considered in the design of the filter and surprises appear.
EMC radiated problems in HF/VHF ranges are typical in products with switching-mode power supplies (SMPS) as for example flyback and buck converters. Usually no high power is needed to radiate signals well over the legal EMC limits.
Resonances in components are a well-known topic for electronic designers when working in high frequencies (e.g. EMI/EMC). Do not forget to test your components with an impedance analyzer, especially if they are custom magnetic components.
Crosstalk is a well-known problem for EMI/EMC and signal integrity designers. But, for non-specialist electronic designers, this problem can create unexpected and difficult to discover failures in electronic products.