There is a disastrous File Save (mis)behavior when an AI file is opened using "File > Open As Smart Object".
When editing is complete, the expected behavior on Ctl-S or File > Save is for Photoshop to open the "Save As" dialog, as it does when e.g., a jpeg has been modified in such a way that it is no longer a valid jpeg.
What actually happens is that the PSD is saved over the AI file, using the same file name including the .AI extension. The result is an "AI file" that is actually a PSD. It opens in Illustrator as a raster image. The original AI file is gone. This is a disastrously bad bug.
The only workaround is to revert the Photoshop document to its Open state, open the Vector Smart Object (which is identical to the original AI file) and save THAT with the original file name, but this definitely is the wrong behavior in this situation. I'm amazed no-one has run into this earlier (including me); it's been in the program since at least CS5.