Random "Message not downloaded"

[AC Question]

I noticed a notification today on my lock screen with the exclamation point that said "Message not downloaded", and when I clicked on it, it redirected me to the screen where I type a new text.

I've never had this happen before, and it's only done this once so far. I wasn't trying to download anything, and I haven't had any problems with receiving pictures or anything. I can't seem to find any answers, no matter where I look on Google.