I run into this situation all the time;
I often just copy layers and groups of layers in the TOC of one Map Document and paste then into the TOC of another Map Document.
For the most part this works fine but often enough the Data Source of layer changes when pasted. As a result the pasted layer has the red exclamation mark beside and no data is displayed until I manually set the Data Source.
I confirmed that the Data source of the layer I copy on one document changes when I paste the layer into another document.
This is something that has been happening since the early 8.x version and still an issue in version 10 SP3. It's also not computer specific as i run into this issue on several computer.
Layer after being pasted:
Source "layer being copied" Data Source:
Destination "after pasted in another document" Data Source: