This is the explanation from the blog post:
Miscompilations that can arise from the bugs in incremental compilation generate incorrect code in final artifacts, essentially producing malformed binaries, which means that in theory any behavior is possible. In practice we are currently only aware of one particular known miscompilation, but bugs due to incremental are notoriously hard to track down: users frequently simply rebuild after some light editing if they see unexpected results from their binaries, and this often causes sufficient recompilation to fix the bug(s).
If there's some confusion about what this means, it'd be great to clarify - could you be more specific about what in that paragraph didn't make sense?
It's indeed true that check and debug builds that compiled cleanly may have miscompiled.