I just experienced something like this last night. I went on a bus and and tapped my Presto card (green, vertical, non-braille) but it wouldn't read it, so I asked to use the back one. While walking back, someone behind me successfully tapped, I tried to tap on the back one, didn't work. So I just used my spare one and it worked. Then I thought I came to the conclusion that my card was broken, but I used one of the reload machines to check balance and it work, and it word again this morning and afternoon on GO. Maybe I was just not putting my card close enough or I was just not thinking last night.