Coax characteristic impedance measurement
When you connect to a 50 ohm device, you usually make the connection with a co-axial connector. 

Alligator clips, and the tails of wire leading up to them, will introduce a section of higher impedance. 

Whether this makes a negligible or a significant difference depends on your operating frequency, and the length of the tails. If you keep them well below 1/10 of a wavelength long, you'll probably get away with it for power transmission, though not for precision impedance measurement. Once they're approaching 1/4 wavelength or more, they'll really mess with your setup, whatever you're trying to do.