A potion (like speed or slowness) will change your FOV but won't change your speed at all. I encountered this in 12w21a, might be in other snapshots. I couldn't find any posts related to this problem.
Environment
Windows 8, Java 7 Update 21, 64-bit
MC-16484
Potions don't work in 12w21a
Resolved
Duplicate
0
30th Duplicate of MC-16143.