: If this is a final export, rename it to something descriptive (e.g., Summer_Vacation_Final.mp4 ) to avoid losing it among other "xnewvid" files.

: If the file won't open, try playing it in a versatile player like VLC Media Player which supports almost all MP4 codecs.

: You can use the Final Cut Pro X Video Inside Text Effect technique by placing your text over the video and using the "Stencil Alpha" or "Stencil Luma" blend modes.