In [1], we started checking for gen_branch failures, but I made two crucial mistakes. One, defer_compilation() had the same issue as gen_branch() but wasn't checked. Two, returning None from a codegen function does not throw away the block. Checking how gen_single_block() handles codegen functions, you can see that None terminates the block with an exit, but does not overall return an Err. This handling is fine for unimplemented instructions, for example, but incorrect in case gen_branch() fails. The missing branch essentially corrupts the block; adding more code after a missing branch doesn't correct the code. Always abandon the block when defer_compilation() or gen_branch() fails. [1]: cb661d7d82984cdb54485ea3f4af01ac21960882 Fixup: [1]