This is just for the record... I have one particular, occasional client who has a solution originally designed (3 developers back) in pre-7, and since partially optimised. Given that it's a manufacturing company with complex needs, it would come as no surprise that the solution is... labyrinthine. Oh well, a curious discipline of sorts.
The discovery: I had to do a new report recently and I copied an existing report as a starter. Added some field defs with formulae and repointed fields on the report to the new defs. Also created fresh copies of the fields back on the form layout so that I could click through records to check how they were performing. Frustration - poor results. Suddenly I realised that the fields were displaying different results for the same record on each layout. Huh? Must be a clumsy, tired eyes kind of developer error, yes? eg different TOs, etc etc?
Actually, no. It was truly weird. Eventually I set up two copies of exactly the same field - the original 'repurposed' one and a fresh one - on the report layout, and they displayed different results. I have never seen anything like it. I can only hypothesise that the 'old' one - in this oft reworked file - somehow clung on to an erroneous index or something. I don't know if being on a List layout had anything to do with it. Naturally, it was easy to make the 'bad' one disappear forever.
The lesson to take away: usually if there's a really wacky result, it'll be a developer error. But just very occasionally, if your gut tells you that your calc is correct, it's worth digging a little deeper and checking for the highly unlikely possibility.
Note: With no urgency, I'm open to suggestions of an off-the-shelf, customisable package that might better serve them into the future. They're not planning on this right now, but I'm willing to start pitching something that's a few generations beyond what they have, as it's getting increasingly hard to support old architecture (even tho' it's study and fundamentally well-designed). It'd probably end up being a 6 to 12 month decision cycle - quotes, jobs, production tasks and time, materials, contacts, invoicing and reporting/ planning.