Many interviewees tend to forget to prepare ahead of time before an interview - some do, but some don't (prepare enough). It can be really stressful preparing everything beforehand, but it will ease your mind knowing that you have everything planned out before the day of.

Most Commonly Forgotten Things To Prepare

Some of the most common things interviewees tend to forget are what they will wear, their interviewer's name, and their research on the company they are interviewing for.

Clothes & Attire

Interviewees tend to forget about their clothes because they think that they will look good in whatever clothes they bring with them, but when the day comes you might get dressed up in something way different than what you had planned to wear.

Investing in new clothes is an easy fix if you forgot about it, but sometimes it can take days or weeks to find what you need/want.

Tie or no tie: Male grooming guide for every kind of interview
Be formal and there you have a good impression. Be too formal and awkwardness sets in. Trying hard to fit into the interview setting? Sure - here’s how you square up your situation.

Some say the clothes make the man.

You don't want to rush to the departmental store at the last minute to replace a moldy tie or taper your skirt so it fits right.

Prepare your attire at least a day in advance, and hook the "interview-day costume" on a corner of your closet so you don't panic over these little things at the last minute.

Well, I think it also helps you stay calm and composed so you perform at your peak during the interview. Also consider reading this article below as well.

How to Calm TF Down & Relax Before An Interview
Stay calm on the way to the interview | Get sleep | Eat a healthy breakfast | Prepare answers to common questions | Show up early enough

Interview Details & Interviewer's Name

The other common thing interviewees forget about is the name of the interviewer. Unless you are talking to someone who has personally interviewed someone at your company, then their name isn't important information for this discussion - it will be brought up later on.

Preparing a script

Having your sentences, thoughts and ideas planned out ahead of time can greatly reduce the stress that comes with interview preparation. Memorize some salient ideas which you can recall and regurgitate (i.e. spit out) during the interview can make on-the-spot thinking easier when you are on the hotseat.  

Getting your script right, getting it "all planned out" ahead of time will ensure you do not "say the wrong things" which will ruin the interview.

Saying The Right Things During Your Job Interview
What to Say | Beginning of Interview | When You Have No Experience | Self Introduction | Opinion of Ex-Boss and Past Projects | At the End of the Interview

Applying Research into Action

The last common thing interviewees forget about is the research they did on the company they are interviewing for. The most important thing to consider when you are researching a company is if their values align with your own. If you aren't passionate about the company/organization you are interviewing with then it would be best not to join them.

You should apply the research into quantifiable action in the interview.

How To Research A Company Before An Interview
Start With a Standard Wikipedia Search | Use Linkedin to Research on Key People & Their Connections | Social Media - Public Updates & News, Customer Sentiment | Corporate Website

Conclusion

These are the three most common things interviewees tend to forget, but there can be many other things that interviewees tend to forget. It might be really stressful preparing everything beforehand, but afterwards you will feel relieved knowing that you have everything planned out before the big day.