Do not make anything harder than it needs to be. Unless something is a sin, keep promises to their spirit. Nothing more, nothing less.
In general, anything done by mistake is not a sin. As long as you were not doing or watching anything sinful that led you to do or watch what you promised that you would not, it is not a sin (for example an ad popped up when you were doing homework for school):
"Verily actions are by intentions, and for every person is what he intended. So the one whose hijrah was to Allah and His Messenger, then his hijrah was to Allah and His Messenger. And the one whose hijrah was for the world to gain from it, or a woman to marry her, then his hijrah was to what he made hijrah for." (Bukhari & Muslim)
As soon as you are led into it, you must quickly remember your promise and leave the sin immediately. But, the important part is you were not doing a sin which caused you to break the promise.