Student or Learner
I doubt where an "it" should be added after the word "realized". Also, please correct my mistake if there is any. Many thanks!
I mistakenly took the deadline as 12:00 at night and only realized after the notification.
I am not a teacher.
I mistakenly thought the deadline was midnight. I realised only after receiving the notification that it was actually noon.
Note that, grammatically, your title should have been "Are there any mistakes in this sentence?" However, thread titles should included some/all of the words you are querying so your title should actually have been something like "I mistakenly took the deadline as 12:00".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.