Flash "obfuscators" (you'll shortly see why I put that in quotes) do this. The real AS code, unmolested, is stored in an unknown tag type (I've seen 253 quite often) right before the DoAction, and the DoAction itself contains the obfuscator's code (usually a "jump forest" sort of thing). The intended behaviour is for the obfuscator's code to be executed, then it jumps out of the DoAction and into the one before it, and there's a jump there which then takes it to the real entrypoint of the original code somewhere near the beginning of that unknown tag.

I've recently tried to use Gnash for playing Raft Wars game (Flash 8)

http://armorgames.com/play/990/raft-wars
(http://www.armorgames.com/files/games/raft-wars-990.swf)

I found that when developer's splash screen disappeared, frames in the game
became uncontrollably looped, as if there's no actionscript interpreted.

While the frames were looped, Gnash repeatly printed lines like these to the console:

4528:1] 1814 UNIMPLEMENTED: Jump outside DoAction tag requested (offset 16 before tag start)
4528:1] 1814 UNIMPLEMENTED: Jump outside DoAction tag requested (offset 16 before tag start)
4528:1] 1815 UNIMPLEMENTED: Jump outside DoAction tag requested (offset 16 before tag start)
4528:1] 1815 UNIMPLEMENTED: Jump outside DoAction tag requested (offset 16 before tag start)
4528:1] 1816 UNIMPLEMENTED: Jump outside DoAction tag requested (offset 16 before tag start)
4528:1] 1817 UNIMPLEMENTED: Jump outside DoAction tag requested (offset 16 before tag start)

These lines would repeat endlessly (together with looped frames) until I hit Ctrl+C to exit Gnash.
Full log file (verbosity=1) also attached here.

Gnash: gnash_0.8.10-1~lenny from getgnash.org APT repository
System: Debian GNU/Linux 5.0 Lenny i386