In Brussels I needed to get one of the metro police to let me out; thankfully, they're at nearly every station downtown. With TTC the station staff would likely handle it; another option is the Washington DC/BART method of putting ticket purchase machines in the fare-paid areas.
Fun fact, I lost my ticket inside the faregate. There was a gap between the electronic tap surface and faregate body plenty large enough to suck in one of those bendy paper tickets when I slide it off. Gate was open, ticket was gone, I went through anyway (gambled I wouldn't need the ticket to exit; was wrong).