Inventory lag was improved in 13w04a, but not entirely resolved. In the current release of 1.5, when an item is clicked on in the inventory, it will take about 1/2 a second to react.
Can also confirm (you should update the affected versions). From my tests I have also seen that the quicker you tap the W key in succession, the more likely it is to occur.
Jack O'Connor
Inventory lag was improved in 13w04a, but not entirely resolved. In the current release of 1.5, when an item is clicked on in the inventory, it will take about 1/2 a second to react.