Alter of stored procedure and execution performance
Had a situation where a production environment had DB schema updated
The developer executed an alter on the base procedure, with no changes, and the execution time returned back to normal and they asked why.

Certain changes in a database can cause an execution plan to be either inefficient or invalid, based on the new state of the database. SQL Server detects the changes that invalidate an execution plan and marks the plan as not valid. A new plan must then be recompiled for the next connection that executes the query.
From BOL - Recompiling Execution Plans :