While I really love the MC-909, I must say the machine has several bugs, and being discontinued by Roland, no further fixes will be coming, i'm afraid.

-SMS IMPORT BUG:
One of the major ones I have found AFTER OS 1.29 has cleared the old ones, is that you CANNOT import a Midifile to a pattern you were working on already. If you do, it will reset all of your work, including sounds, mixer settings, effects and everything else you set in that pattern to the factory settings. How convenient.
If you are working on your computer and need to bring a sequence into the 909, proceed as follows: select an unused pattern you'll use for that purpose alone, import the midifile (with the usual procedure) into that dummy pattern, then use the COPY function (all tracks at once, obviously) to bring the sequence from that dummy pattern to the one you were working on: click EDIT, then COPY; a window will open, click SELECT, then another window where you choose the destination pattern, then EXECUTE. After that, hit EXIT, then WRITE, to print what you just did to the real working pattern. The sequence data will now be into the pattern you were working on, and all the mixer/patch settings will be intact.

-TEMPORARY EDITS BUG:
the machine will forget all the mixer/patch/effects edits you have recently done if you do some sequence edit on that particular pattern without saving beforehand. So make sure, before you Copy, Erase, Delete, Move or whatever else you want to do to the sequence, that you WRITE the pattern beforehand or, after the sequencer has done its work, all your sound/mixer settings will revert to yesterday morning.

-DRUM MAPPING:
well this isn't really a bug, rather an engineering overlook, but it's THE MOST ANNOYING quirk of the 909.
This machine has its drums patches mapped in its own particular fashion, making it impossible to import any drum sequence you have created elsewhere, and Standard Midi Files too. You'll need a computer soft-sequencer to re-map all the sounds BEFORE you import the sequence into the 909, and even then i'm not sure of the results, unless it's a very simple kick-snare-hhat sequence and you can get the mapping straight away. Add percussions, and you're screwed.
Honestly I'm not even sure you could create a GM mapping on the 909, because it appears you cannot identify single keys in the way Standard GM mappings suggests, and also drum patches are only 2 octaves long, and are muted below and above that; i didn't really look that far into drum editing as I have little patience, but i believe it might be really difficult to imitate some GM mapping on it.
Did anyone ever attempted this GM mapping? let me know.

-TURNTABLE SLIDER:
I strongly suggest to anyone doing serious live work on the 909 to DISABLE the turntable slider, as any slight movement could cause pitch or bpm to go off, unless you dechecked their switches, and, in the heat of the moment, many could forget those. I have blocked the slider with a small bracket that holds it at its uppermost position, but you can use tape, adhesive stickers, whatever. If you're into tech stuff, you could even disable it from within. Besides, no serious musician or DJ would ever need such a amateurish feature anyway.