If you don't touch in or touch out on London tube, your journey will be incomplete. Because of that, the right fare can't be charged, so you will be charged the maximum fare or penalty fee.
The maximum fare depends on if you travel during a peak or off-peak, but it also depends on area. Maximum fare doesn't count towards capping, so you can be charged quite a lot in total.
Please mind there are maximum journey times, so you have to touch out within a certain period of time.
In Zones 1 - 4, when travelling on Monday - Friday, between 04:30 AM to 07:00 PM, the maximum journey time is 1 hour 50 minutes.