That assigning a priority other than "1" means the task will never be done.

I had a manager ask me to customize the bug-tracking tool with priorities 1a, 1b, 1c, 1d, and 1e so he could assign everything as some variation of priority 1, but we could still organize the work.

And yes, I did get through all the "priority 1" issues. But before I could proceed to the issues that were still in priority 2-5, the manager made me re-assign the priorities of those issues into the five priority 1 levels.

(I realize this myth isn't specific to programming, but that's true of several other answers on this thread.)