A temporary lapse of memory when something you normally know slips your mind (e.g., a name, a word, or a kanji you can't recall for a moment).
会議で相手の名前をど忘れしてしまって気まずかった。
I had a momentary lapse and forgot the person's name during the meeting, which was awkward.
漢字がど忘れで出てこないときはスマホで調べることにしている。
When I can't remember a kanji in a momentary lapse, I look it up on my phone.
大事な場面でど忘れしないように前もって練習した。
I practiced in advance so I wouldn't have a memory lapse at an important moment.