A handful of recent threads on Apple's Support Discussion forums - backed up by our own testing - indicate that Snow Leopard suffers from a bug that can prevent users from opening multiple recently downloaded files simultaneously. Upon attempting to open a group of downloaded files (that have not been opened previously) from the Finder via double-clicking or drag-and-drop, only a portion of them open in whatever application owns them; no error message is displayed. If all opened files are then closed, and opened as a group again, one more file will open than on the previous attempt. Each successive attempt to open the same group of files will result in one additional file being opened, until they are all opened successfully.
In general, opening multiple files from within individual programs works properly; remember that in most applications you can select multiple files in Open dialogs with Shift- and Command-clicks.
Variations in the reports of the bug do exist. Some users found that the issue surfaced only after attempting to open a certain number of files. Others found that no specific threshold existed, but that any attempt to open a group of files would succeed for only about 60 to 75 percent of the total number of files.
Most users in the forums complained about experiencing the problem with image files (in JPEG, GIF, and Photoshop format) and PDFs, though Word documents were also mentioned. For those experiencing problems with image files, some found that all image file formats were equally affected, while others reported that certain formats appeared immune. Also, while a few users claimed that files that had been opened before were affected, the overwhelming majority of users reported otherwise.
A user writing on the Mac OS X Hints forum suggested, after having explored permissions on affected files in Terminal, that the issue is related to files being flagged by the system as quarantined. Of all the suggested causes, this one has gained the most support, and others have posited that Snow Leopard's stronger security settings are at the root of the overly restrictive quarantine attributes.
Since the quarantine theory emerged, some command line-based workarounds have been offered on the Apple Support Discussion forums to un-quarantine files. If you are unfamiliar or uncomfortable working in Terminal, and you are trying to open image files, consider changing "Open groups of files in one window" to "Open all files in one window" under Preview's Preferences; at least one user found this to be an effective workaround.
As always, if you are affected by this bug, consider adding your experiences to the ongoing forum threads linked earlier or contacting Apple (either online or by working with an Apple Genius at a retail store) to voice your concerns. We can hope the forthcoming Mac OS X 10.6.2 will fix it.