I've encountered a MIDI bug that manifest itself as playing notes that aren't visible in the item. This has happened to me once before, but this time I isolated the bug to a Reaper project that I can share.
NOTE: a workaround is to open the MIDI item, copy all the notes and paste them into a newly created MIDI item.
I've attached a project that reproduces the bug in v5.70/x64 (Win7-64 SP1). It contains a single MIDI item which appears empty:
However, if you play this item, there are notes.
If you scroll the arrange view to the left, so that left edge of the item is obscured, the notes become visible:
Even more bizarre, if you slide the item left, the notes don't move with it; it's like a window being dragged along stuck notes. But then if you move to the item to the right, the stuck notes appear to reattach to the item.
(if that doesn't show up as an animated GIF, click
here)
That's it. Project file is attached. The buggy item is at 87.3 on the timeline.