What should I do if I attended a conference but forgot to register? Due to some unexpected personal events, I left the conference earlier than intended. The lines to register had been so long and from past experience, I knew the trick was to wait until the last day when there are zero people in line. It's been two weeks and I was filling out my travel reimbursement when it hit me. I never registered...

I presented a paper and chaired a panel. I feel terrible about it, but at the same time, I'm a brand new assistant professor and my student loan payments just started and well, I'm flat freaking broke.

I was just going to say screw it and be sure I pay next year and also sign up to volunteer for some things next year. But now I'm really concerned that my university is going to ask me why my registration isn't on my travel reimbursement form. I don't want to give them the impression that I did I'm the kind of person that would intentionally sneak into an academic conference without paying.

What should I do?
I would suggest to contact the conference organizing committee as soon as possible and ask them what can be done about it. You can explain what happened and why, and also directly offer to pay the registration fee afterwards. If you presented at the conference and also chaired a session, there should be enough evidence that you were actually there. Conference committees are human too, and will understand that mistakes happen. If you are honest, it should be possible to convince them that you did not intentionally try to sneak into the conference or avoid the fees.

Note that many conferences will only publish your contributions if you have a valid registration and also paid the fees, so it might definitely be in your own interest to fix this mistake.

As the comment by @Roland suggests, it could also be helpful to contact your administration responsible for the reimbursement and to ask for their advice on how they want the situation to be handled. Since you are worried what impression you might give them, this should rather help to have them understand that you actively try to correct your mistake.