I've seen this happen a couple of times recently, when I give positive rep the member's rep count seems to either not change or go down. See this post for example. Then look at the reps give in his profile page. The one I just gave him isn't there. Also see the line chart -- the line for rep count went down. Does that indicate he got some negative rep?

I have also noticed the same in the past.

Is there any restriction on how many up or down votes one can hit in a day ?

The line for rep count on the profile gets connected to zero if you haven't gained any rep yet that week, it doesn't mean there's been anything negative(if it had been negative, I think the entire graph shifts up so it's noticiable). In any case, I only saw an upvote when I went to look at that thread.

Is it possible you've reached the daily limit (I forgot how many times you have to give rep, but I seem to have reached it on occasion)?

I didn't realize there was a daily limit, but I doube that I've reached it.

There is a daily limit, but a popup error message should show up if you've reached it. I'll investigate that. However, the thread you linked me to only has received votes, no reputation.

What the difference between votes and reputation.

a popup error message should show up if you've reached it

I've never run into the error message. I've only known because the "View Reputation" link didn't appear next to the arrows and I was sure that I hadn't given that person reputation already that day (and any subsequent attempts resulted in the same thing). I can't remember how long ago this was, mind you.

Ah yes, I probably changed it at some point from an error message to the arrows just not appearing at all.

What the difference between votes and reputation.

Votes on a post give a +1 or -1 to that individual post. And that's it. If you leave a comment when you vote, then you also affect a member's reputation, based on your own reputation score.