Thanks for the links - I hadn't found them despite some creative Googling. It appears from the 1st one that it's possible to reproduce this. To be honest, given that it's occurred in our live environment, I need a solid workaround/mitigation for it - it was denying access to approximately 8,000 users, due to the fact that the SP was being run at login!

I think I'm going to have to put in place a proactive sp_recompile for at least this SP, if not all, after the index defragmentation. This will cause a small performance hit when each SP is next initially run.