I couldn't find the thread where this was originally mentioned, but to summarize, someone doing narration was hearing occasional, quiet clicks after rendering but not during playback. I also don't recall that he got an answer, but I had something similar happen last night. It was driving me nuts because I would keep hearing a click after rendering where seemingly, no audio material existed.
 
It turned out the clip was so short it was no wider than a grid line (!). It was not visible and the only way I stumbled on it was passing the mouse over a (supposedly) blank track, and the slip-editing tool appeared. As to why I didn't hear it during playback, the only theory I can come up with is that it was so short it blended in with the tracks, but when rendering a finished track which included a limiter in the output bus, it "tripped" the limiter. 
 
I found a couple other tiny slivers like this. As to why they existed, I do a lot of cut/paste/move etc.  Sonar defaults to snapping to the nearest audio crossing and usually, I don't want that. However, I don't always remember to uncheck it until after I've worked with the program for a while. Maybe I somehow ended up cutting in such a way that a little sliver of sound was left over after cutting at a snap point as part of a mass cut instead of at a zero crossing.
 
In any event, after I tracked these down and deleted them, the problem went away.

I had a sliver like that in Beepster Creep and if I try to do ANYTHING with it in X2 (where the project was created) like slip edit it, delete it, move it, etc it would crash Sonar. It didn't seem to make any audible sounds that I could tell so I just left it alone. Then when I upgraded I tried to open the project in X3 to finish mixing and play with all the new goodies but playback would stop EXACTLY at the spot where that little sliver was. There was absolutely nothing I could do about it so I had to finish the project in X2.
 
It was really freaking weird. Damned gremlins.

One possible source of the slivers could be the comping mode, I sometimes will split a clip in a take lane so I can edit out a string noise that may be present in all three takes. If you're not careful when trying to slip edit you can create additional take clips, (not in new lanes).
 
And further more they seem to be underneath the clip you've slip edited.  It's still all a little new to me and I can't confirm a recipe nor that these clips cause a clicking. Simply an observation of something that has frustrated me.

For grins I made a one-sample long stereo clip with the left channel sample at -12dB, and the right at -Inf. The pop in the left channel is clearly audible on playback even at very low monitoring levels, and nulls with an inverted bounce as expected.
 
All other things being equal, this should always be the case no matter how short a clip is or what the buffer size is. As John pointed out, it would be concerning if it were otherwise.
 
 

Just to follow up - it does seem related to the limiter in the output bus being "slammed" rather than the shortness of the clip. I should probably delete this thread so no one gets let down the wrong path.

Anderton
Just to follow up - it does seem related to the limiter in the output bus being "slammed" rather than the shortness of the clip. I should probably delete this thread so no one gets let down the wrong path.

I'm not sure about deleting this thread, personally.
 
Maybe change the title, but deleting it would also lose some useful information.  The orphaned "slither" has caused me problems in the past.
 
Just my two pennys worth.

Good point. Instead of deleting, I took out the material that could have led someone down the wrong path, changed the title, and left the useful stuff in.

My concern is that the idea that we can't be sure about playback absolutely mirroring render is a huge problem. Clearly I don't think Craig had that in mind when he posted this thread. I think he was trying to solve a problem with clicks and pops that some few have reported. 
 
However the ramifications of saying that playback can miss things and render may not is extremely troublesome. 
 
Therefore this is a very important thread that needs to stay put and hopefully a good resolution will come out of it.
 
I'm hoping to hear from CW and members that can shed light on this.  

I've had these slivers appear from time to time. Have never worked out where they come from.

They've never given me an audible problem, though.
 
Sixfinger's post above sounds like a very likely cause. Must watch out for that. I can't remember whether this is just a post-X3 phenomenon, though.