So just from experience I keep two tickets in hand and in the case one doesn't work. This problem I only face when I have bought 10 tickets ballet never with individual tickets but then if you have a valid ticket and no access, you can always go and ask at ticket counter and they will check your ticket and let you enter or will just renew the time-stamp on your ticket if that's the issue. But is convenient in a way time-saver (of course not when it doesn’t work) and most of the time there is always a queue at ticket windows or machines.
Easy thing is to buy from some small metro station as it won’t be that crowded, smart huh? ;)
Ok enough metro. I am back to home over the weekend and time for some luxuries.
Have a nice weekend everyone.