I've only ever officially participated in one. I'm still going ape craze from time to time about it. It's been running for 3 years.EDIT:
making that a more useful answer...
The game used direct access to files from the disk without using file structure info, which caused all sorts of problems to be handled. Once that was out of the way (i.e. 5 months later), there needed to be a variable-width font support, support for ASCII instead of "SJIS + sometimes ASCII" (and like other people, I've seen multiple functions implementing strlen in various ways).
I've replaced the original font which looked "sorta OK" (but mostly crappy) and used more horizontal space than I wanted it to. (I've had multiple request from my favourite translator to have a more squeezed font, but have pushed back because of my own personal taste.
Then of course, like others, I realized this completely messed up text in all menus, because text alignment didn't take into account the VWF pixel width, the texts were longer, some text alignments weren't done properly in Japanese anyway, some screens just had to be shuffled around because they wouldn't fit otherwise, some texts were located thanks to knowing a space is the same width as a character in Japanese, etc.
There was also the need to interpret data structures for menu text, because we needed to have the ability to add more lines to encyclopedia descriptions, and those are in a table of indexes to a table of pointers to a table of texts. And we got many of these...
I've had to interpret the data structures for story text, so that we could add more dialog boxes (even from a different character that the one originally speaking, if needs be.)
Text spoken during battle animations followed a different structure, so those had to be understood too.
Other stray texts dwelled in the main executable itself, so that took some more asm changes (easier done with the use of armips).
Graphics were not all stored in the same way or place, thus I've had to edit 4bpp and 8bpp graphics (after finding them), as well as some compressed graphics.
I've had to design a logo, because the existing SRW ones were either in Japanese or lower resolution.
I've had to design an editor that would help translators and editors by auto-wrapping lines, in a way everyone likes, applying a set of (in the end) 6 rules o decide where to wrap and where to create new dialog boxes. And we discussed some more enhancements, of course. The tool also had to warn us when lines are actually "weird" (ex: new dialog box created with a single word).
We ran out of RAM at some point, but a good suggestion to "cut" unused font memory solved that.
Luckily, I didn't have to deal with several compression levels, many many files, and such.