I've landed twice without a clearance. (And I'll readily admit it.)

First time was with a student... we were cleared for a touch and go. We'd requested the option and I didn't catch the controller clear us for the T&G. Ended up making it a landing and realized it after we cleared the runway. Fortunately no loss of separation and no other planes behind us, so no harm no foul.

Second time was flying into Syracuse, NY. Things got busy for both us and approach due to low weather, holding at the marker and runway swaps due to snow removal. Realized that we were still on with approach as we cleared the runway (oops!). Switched over to ground and fessed up (Can you say ASAP). Ground was very cool about it and replied "Yeah, we thought you were having some radio issues. You saw our light gun signals right?"

Nice when folks work together.

I've made a point of it to double check clearance at the end of the before landing checklist- "Three green, cleared to land, runway 1L". That way, you're not catching the issue at 100 AGL and can get in a quick- "Airline 1234 cleared to land?"