I helped a woman out that was having trouble with a Somerville parking meter. There were two problems:
1) There were two numbers above her parking space. Only one was the parking space number accepted by the meter.
2) (This was the critical one) It informed the user that a parking space was not valid via message that flashed by for literally less than a second before returning that space on the display to unhelpful promotional text. So, we had to try a couple of times in order to read it.