As per title. RFTools v7.15 has a hard dependency on XNet, causing the game to fail to load if XNet is not installed.

EDIT: Relevant log section:

[17:49:30] [Client thread/ERROR] [FML]: The mod rftools (RFTools) requires mod versions [xnet@[1.5.0,)] to be available
[17:49:30] [Client thread/ERROR] [FML]: net.minecraftforge.fml.common.MissingModsException: Mod rftools (RFTools) requires [xnet@[1.5.0,)]

Hmm that's strange. You don't have an older version of XNet in your mod list?

I thought I had removed XNet some time ago, but it seems I forgot to remove it from the folder I download my mods to. Derp.