The one that keeps coming up is how 'code' is rendered. Old reddit uses four leading spaces for a code block and IIRC new reddit uses triple-backtick. It's probably slightly different, not-quite-100% compatible markdown dialects.

Admittedly it's a problem more common on the more technical subreddits.

Ah. I have noticed that one actually, but I chalked it up to someone confusing Github/SO/Reddit markdown. Didn't realize that Reddit itself was inconsistent.