This is not documented in the ref below post form, but Markdown (which WL uses) supports headings. There are two way to create them:
1. Put a line of dashes or equal signs below line of text:
Stuff
----
Stuff
\====
(this appears not to work correctly. The visible backslash is not my fault)
Stuff
Stuff
2. Put a hash (or - up to 6 hashes) in the beginning of line of text (this is what happened):
# Stuff
## Stuff
### Stuff
#### Stuff
##### Stuff
###### Stuff
Stuff
Stuff
Stuff
Stuff
Stuff
Stuff
To show the hash, precede it with backslash \.
One more quirk worth mentioning: Markdown assumes everything that looks like "[digit]." to be a beginning of list - thus for example 2. will render as 1. (No, really.)
Fix: precede the dot with backslash, like this: 2\.
To show the backslash, precede it with another backslash: \\.
More quirks and detailed guide: http://www.reddit.com/help/commenting (some may not be applicable, reddit seems to use slightly different flavor)