A recent Hackaday post describes the construction of a quarter-wave impedance matching circuit. The author was asking why his system shows a resonance at around 600 MHz.
Typing his circuit into SimSmith shows the following results.
The first image (above) shows the SimSmith simulation of his original circuit. The green line on the Smith chart shows that a 90 degree length of his 41.56 ohm cable will indeed match the 25 ohm load to - approximately - the 50 ohm generator. The line actually lands on 69.1 ohms, which reflects the fact that he's using a constructed 41.5 ohm line and not 35 ohm line (which would make an ideal match).
This next image shows SWR plotted as a function of frequency. His calculations indicate that the minimum SWR should be around 200 MHz, but was confused when his NanoVNA showed a minimum at 600 MHz.
The image above shows what is actually going on. I believe he's seeing the 2nd harmonic of his calculated frequency, and SimSmith indicates that there should be a 3rd harmonic as well.
SWR is not 1, which reflects the fact that his impedance match is not perfect - 69 ohms instead of 50 ohms - because his constructed transmission line is not the optimal value.