Minecraft 1.4.2, Minecraft 1.4.5, Minecraft 1.4.7, Minecraft 1.5, Minecraft 1.5.1, Snapshot 13w16a, Snapshot 13w16b, Minecraft 1.5.2, Snapshot 13w19a, Snapshot 13w21a, Minecraft 1.7.4, Minecraft 14w03b, Minecraft 1.8, Minecraft 1.8.1-pre3, Minecraft 1.8.1-pre5
The silk touch enchantment doesn't activate on the very last use of a tool. The tool just breaks and what should normally happens (you get glowstone for example) doesn't happen (You get glowstone dust instead)
How to reproduce :
Step 1 : Enchant a tool with silk touch
Step 2 : Reduce the tool so that it has 1 durability left on it
Step 3 : break the tool on something that gives you a different thing with silk touch
Result : You do not benefit from the silk touch enchantment
I am unsure if this is also the case with fortune.