Image?? What image?? Could the doctor perhaps have been talking about a different patient, nearby? The spinal tap -- it's like a needle inserted into your spinal column, and fluid drawn out into a syringe. There is no "image" there to be taken??
Hospital mix-ups seem universal. I can't begin to tell you about the number of times we've run into the same sorts of screw-ups, over my husband's various illnesses this last year. Appointments set and broken (and we've not been told), appointments in clinics different from where we went the first time..but nobody told US about that!... different doctors because the other one quit in mid-term...and other crazy stuff.
about all you can do is what you did -- take someone with you who can help sort it out, and just sort of go with the flow.
I'm glad you avoided the headache. Yes, the 4 hour wait is sometimes part of it. -- they want to be sure you don't have the headache, and if you do, that they can treat it immediately.
I hope the evoked potentials get done with less stress for you! It's a good idea to call ahead...but as I've found out, even that's no guarantee that all will be in place when you get there!
...I am not a doctor, nor health professional, and don't pretend to be one, here.....