Tab key sometimes fails to autocomplete in Go To Folder… sheets in Mountain Lion I like keyboard navigation, so I frequently use Shift-Command-G to Go to Folder…  when saving files. I type the first few letters of the directory I want then key Tab to complete the rest. This worked consistently on my old MacBook Pro with Snow Leopard. 

On my new one with Mountain Lion: 


tab completion works about half the time
the other half, keying Tab is ineffective. 


I've observed this with Finder, and with Save dialogs in TextEdit and Firefox. I've carefully verified the spellings, and paths to the directories. 

I've also seen it go from not working to working without my doing anything noteworthy in between (didn't reboot, quit and restart the program, or quit any other programs). 

Added information:


The asynchronous progress indicator is not spinning while this happens. 
This is not an issue only with ambiguous tab completions (when there's an ambiguous stem, it usually just autocompletes with the first match alphabetically.
This does not involve files on an external drive.
If I type in the full name of the directory I'm trying to autocomplete, the "Go to Folder" dialog will not go there (it acts as if I typed in a nonexistent folder name). However, I can still navigate there manually using the save dialog.
Nothing appears in the Console or the system error log at times when this error occurs. 
At Graham Perrin's advice, I ran mddiagnostic, but I have no idea how to interpret the output beyond the system error log I already examined.
If the asynchronous progress indicator spins, in the sheet, immediately after you key Tab then something is ongoing and you can wait for the operating system to get what it needs from the file system. 

For example, a wait of maybe seven seconds between the following screenshots: 






If the problem is without a progress indicator, then see whether the 
Console view of all messages presents anything of interest at the time of the problem. 



If nothing of relevance is visible in Console, then you can run mddiagnose to gather information, some of which may help someone to diagnose. 

mddiagnose(8) OS X Manual Page