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What is the most forgotten letter in Happy Birthday?

I just found out that when making a birthday cake and writing "Happy Birthday" on it, there is a certain letter that is typically forgotten. I found this out because my birthday is tomorrow but we celebrated it today, and the person that made my cake also made this apparent very common mistake. Do you know what the letter is? Also tell me some birthday horrors from the past that you have experienced. First one to guess the right letter will get chosen as best answer.

Public Comments

1. I'll guess the "h" in Birthday. By the way, Happy Birthday!
2. The most forgotten letter is the thank-you note.
3. The H in Birthday?
4. One of the P's in Happy. i dunoo! ^_^
5. Interesting question. (I have not looked at any ones answers.) I'm guessing maybe the "h" in Birthday? And, Happy Birthday to you! (That's the part I forgot when answering this the first time!!!! ha ha ha)