Stacking heat was in fact a bug. You were never meant to do it, it was causing the server to crash (the reason they took out skegs, until they finally took out the bug). It fell under the rule that goes something along the lines of, "abusing a glitch to gain large amounts of zeny". If one person could easily solo Valk, there was definitely a problem. Offical RO servers have patched said problem too.
There were other glitches, too. Like the lutie cake glitch, which let people making those cake foods and sell it at a lutie tool shop dealer for insanely high amounts of zeny.
Don't forget about ad frauding and how crazy that was...
And now, all I hear is, "We need a money sink." .. Makes me laugh, really.