Something weird is happening

So I reviewed my notes on HTML today and encountered a very weird error message.
I attached pictures with the message I got highlighted in yellow bellow.
The parent block is about using quotes in HTML tags. Indented below that is the block with the error message in yellow.
Thing is, I don’t remember putting large information in one specif block like that. Also that block contains information unrelated to using quotes, so I would never put anything like that under that block myself. What’s even weirder there is text that I don’t recognize at all like some Japanese characters at the very end.

But that’s not all. There is a delay between expanding and collapsing a block, not just the block with error message, but all others. It was not like that when I started today, but only after I opened that block with yellow message.
There was also one other similar block I encountered today, but I just deleted that.
Anyone knows what’s going on?
Could I be hacked or something?