Here is some general information
1. Offices have different standards. The offices I ran had extremely high standards, and strove constantly for higher standards. Everyone was expected to improve their methods and means--and did, because they got bonuses for good ideas. The fact that you were told, other files got lost indicates poor office management, which means those bosses were scapegoating. You were probably one in a long line of people, who wasn't trained properly and who they used wrongly.
2. 85% cannot be right--the laws are just too stingent--and no office of size will risk lawsuits. Instead there are "code" words and phrases--meant to clue in other employers without saying anything negative. If someone wants to bury an ex-employee, that's how it's actually done.
3. It works the same on applications. There are code words you can use too. Reason for Leaving:
"Mutual agreement" says there was a conflict, and you wanted out. Another could be "Job description discrepancy" and simply "when they hired me, they thought I knew more than I did." Never say a word against fellow employees, your boss, working conditions, or the company--if you do prospective employers will be sitting there thinking, "She will be saying bad things about me next, if I hire her."
4. Turn your lack of knowledge of computer skills into a plus. Show eagerness to learn and/or to be trained. You can tell employers that you are taking classes, but never show classes on a resume until you finish them.
5. Keep in mind that most offices, use computers like typewriters--and most employees don't really know how to ring the max out of computers. To learn practical applications of computer features, make your own project, e.g. list all your music or clothes or whatever in one program. Learn what all the stuff at the top of the page does. Some is easy, like including pictures, others is a bit more difficult, like export and import, and some is downright confusing like linking between programs, but you can teach yourself every bit of it, if you have the programs on a computer available to you.
Don't get hung up on one bad experience, but also don't walk away thinking the staple didn't matter. It did in my office, because consistancy tells everyone to expect quality work and that makes office life easier. The problem in that office wasn't the staple, but non-training by bosses. Now it's up to you to pick yourself up, dust yourself off, and keep on trucking.