I've created a Maintenance Plan on SQL 2008 R2 Standard Edition with three tasks - one backs up all user databases to disk, one is a maintenance cleanup task which deletes "bak" files older than 2 days, and one is a simple Notify Operator task which should email me if either of the first two tasks fails. There are separate backup destination directories - one for each database on the SQL instance. The backup type is set to "Full" and the server connection is local. The recovery model on these databases is "Simple" as there is no need for point-in-time recovery for the applications being supported. The plan has been running for the better part of a year now and has not been recently changed.
Most days, for most databases, the plan runs successfully and the cleanup task leaves only two backup files in the destination directory (the most current and the day before). On occasion, I'll find one or two databases whose backup directories contain 3 files... It doesn't fail, it just doesn't remove the oldest file. I had assumed this was simply a timing issue. Last night the backup job failed due to low available drive space. When I checked the backup folders, every backup directory contained at least three files and in some cases four files. What could be causing this?