The edition wall
The most expensive mistake at this size is ordering the wrong API product
API access is included by default in Enterprise, Unlimited, Developer and Performance. Professional can buy it, and the SKU is where this goes wrong: Salesforce states that Additional API Calls does not enable API access for Professional Edition, and that you must order the Web Services API product instead.
The failure is silent: the order goes through, the invoice is correct, the integration still cannot authenticate, and two sprints go into suspecting the middleware. Salesforce documents which editions include API access and your account team remains authoritative on what your agreement contains.
Active validation rules cap at 20 per object below Enterprise against 200 above it. Invisible at small scale, and one of the first ceilings a growing business hits without understanding why.
Changing configuration directly in production is almost always a sandbox availability problem rather than recklessness. Allocations differ sharply by edition, so where a team develops is a licensing decision that became an engineering one.