I came up with this unique issue wherein i was receiving this 'Sheet already in use error popup when i was trying to execute my QTP scripts. The QTP scripts were upgraded from 9.2 to 10.0 wherein we had lost the association between External Reusable Actions and the main script which used to call those reusable actions. The Default.xls did not loose Data Tables for the called Reusable actions. We could not help but delete those additional sheets from Default.xls and only then the scripts started running fine. Now even today i still doubt about the upgrade issues we have faced during upgrading QTP from 9.2 to 10.0. I can list atleast 10 issues here and still many to foloow i guess.
But sometimes i feel it was good that we had issues... I get paid for resolving it :)