Mysterious error in flate.rs

[brson]

This has popped up a few times:

rust: task failed at 'Assertion res as int != 0 failed', /Users/user/rust/src/libcore/flate.rs:48
error: internal compiler error: unexpected failure

Here's an example report
servo/servo#58 (comment)

[catamorphism]

I haven't seen this in a long time. Reopen if it happens again.