I noticed in this topic:

http://board.flatassembler.net/topic.php?t=12012

That the code sections have been changed from the original <tab> characters into <space> characters and now the columns are all out of alignment.

What causes this? And can it be turned off or fixed?

This problem appears to affect a lot of older posts. Here is another example:

http://board.flatassembler.net/topic.php?p=93655#93655

The function that processes the [code] blocks has not been changed in years. Maybe something works differently with the modern browsers? I'm shooting in the dark, I did not look for the underlying cause yet.

I know that when they were posted the alignment was fine. But when I look back at those posts they have been changed. What were originally <tab> characters are now stored as multiple <space> characters instead.

It is possible that restoring the database from backup has messed them up. But then it may not be easy to fix it automatically...

BB code parsers don't take TABs unless you use a bot because the tab key event is caught by the browser. Normally all known code parsers will use 4 spaces as a TAB.

Do you mean browser input boxes? You can use paste (CTRL+V) to paste text with <tab>s into an input box.