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Generally, that means ya missed the backslash on the end code </center> , so look for those first.

The other possibility... You may have used another code, skipped a line assuming you could start the code on one line and end it several later... And sometimes you can. 😏 And sometimes it just won't.

For example:

If I wanted to center this
And this
and make it a heading size

but not this
I have to start and end EACH line of the code, but they have to be touching... no spaces.

The above would look like this:
# <center>If I wanted to center this</center><center>And this</center><center>and make it a heading size</center>
but not this
I have to start and end EACH line of the code, but they have to be touching... no spaces.

It's kinda of a bitch. However! I do know there is someone working on a super amazing Markdown editor I'll be testing soon, that will make all this easy peasy.

If you ever need me to look at a post and help trouble-shoot your format, let me know. We can share in real time with Hackmd.io .

Good luck!

That helps. I will experiment with this next time I have problems.