1.5.1, IInventory and AbstractMethodError

I have a class called BaseTileEntity that defines all the methods required to implement IInventory, but doesn't claim to implement it. Then I have subclasses that do claim to implement it, and rely on inheriting the necessary methods from the base class.

When I run the mod unobfuscated under MCP, this works fine. However, when I reobfuscate and run it in a real Minecraft installation, I get an AbstractMethodError from an attempt to call getSizeInventory(). It seems that the reobfuscator is getting confused somehow, and doesn't realise that the getSizeInventory() implementation in BaseTileEntity is able to satisfy IInventory.

This arrangement used to work in 1.4.6-1.4.7. Has there been any change in the way reobfuscation is done that might affect this?

(Searching for "AbstractMethodError" in this forum turns up a lot of references to having outdated mods in the coremodes folder. This is NOT my problem -- it's a completely clean MC installation, and my mod is the only one present.)