Barcelona

Many tourists like the cheap T-10 ticket (10 ride ticket).
It's not really a "10 ride ticket" but a ticket with 10 installments for 75 minutes of travel.
When you exceed one 75min period, another "trip" gets deducted from your ticket automatically once you validate it again at your next transfer.

Therefore you must validate the tickets each time, not only at the beginning of the trip but also each time you transfer from metro to bus to tram etc.
Exception: no validation for transfers within the turnstile-guarded metro system.