It's possible that I have been here too long
We have one core application which is becoming a business risk. It's a Siebel product (Siebel ICM) which Oracle promised to support when it took over Siebel; we use it as a commissions calculation engine. However, since then, Oracle have decided not to further develop it because it competes with Fusion IC, and now they have lost most of the people who knew anything about it, so any time we have an issue it's unlikely that we'll get any help, and if we do it takes months (of course, that's standard for Oracle support anyway). In addition, it's only supported on Solaris 9 and Oracle 9i; all our other production databases are on an Exadata database server, and all our other Solaris servers are on Solaris 10 (and we're trying to abandon them for Oracle Enterprise Linux). There are only two people left in the company who know anything much about how it works.
In yet another chapter of our company's commitment to binding itself ever more tightly to Oracle's smothering embrace, we're starting a project to replace this with Fusion IC; concurrently we'll be changing the ETL tool we use to communicate with it from Informatica to Oracle Data Integrator, so all the interfaces (there are many) will need to be rewritten in a different platform as well as adapted to a different calculation engine. Fun times ahead, apparently.
I started at this company when we were restructuring and migrating commission calculations from a set of Access databases (all together now: TRWTF is Access) into our then-new CRM system, in late 2002. Around 2006 I and a few others implemented the restructuring and migration of our commission calculations from the CRM system into Siebel ICM. (Ironically, the restructuring that time made the structure quite similar to what it had been in the Access databases, though without a fair bit of the insanity that allowed people to do such things as creating a new structure for a given deal just because they wanted it to pay out differently.) Now I (and one of our junior devs) get to migrate the commission calculations again, though this time we're not restructuring them as far as I know. I'm beginning to think that it's possible I have been here too long.
At least it makes for some good stories to scare the juniors with.