How to create and avoid the progress-halting issue, and a last resort fix.
Nintendo of America have confirmed a glitch in Metroid: Other M that prevents players from progressing due to a permanently locked door.
In the Sector 3 area, if after a certain sequence of events, you backtrack to a previous room rather than moving forward through the next door, a certain door later in the area will not open. This does not happen if you proceed normally.
For users who have encountered this glitch, Nintendo are able to fix save data that is sent to them on an SD card. Full details are on the Nintendo website.