The cause of "bad magic number" error when loading a workspace and how to avoid it?

Question

I tried to load my R workspace and received this error:

Error: bad restore file magic number (file may be corrupted) -- no data loaded
In addition: Warning message:
file ‘WORKSPACE_Wedding_Weekend_September’ has magic number '#gets'
   Use of save versions prior to 2 is deprecated 

I'm not particularly interested in the technical details, but mostly in how I caused it and how I can prevent it in the future. Here's some notes on the situation:

1. I'm running R 2.15.1 on a MacBook Pro running Windows XP on a bootcamp partition.
2. There is something obviously wrong this workspace file, since it weighs in at only ~80kb while all my others are usually >10,000
3. Over the weekend I was running an external modeling program in R and storing its output to different objects. I ran several iterations of the model over the course of several days, eg output_Saturday <- call_model()
4. There is nothing special to the model output, its just a list with slots for betas, VC-matrices, model specification, etc.

Answer 1

I got that error when I accidentally used load() instead of source() or readRDS().