I am working my way through a heap of Russian covers and cards from 1921 - 23, a period of high inflation. Postal tariffs were frequently changing, for much of the period gathering noughts. Supplies of appropriate stamps lagged behind the tariffs, leading to multiple frankings and revaluations - and the revaluations were sometimes complex (revalue your kopeck stamps upwards x 100; leave your rouble stamps unchanged)
The task is apparently simple: identify the tariff period and then check to see if the correct postage has been applied, and if not, whether postage due was raised. It involves a lot of adding and multiplying (and as currency is revalued, dividing). It can be fun.
More than that, you realise things like this. (1) Faced with a tariff for which they had no obvious combination of adhesives, clerks applied an approximate franking. (2) News did not necessarily travel fast. The Circular announcing new tariffs may not have arrived or, if it had arrived, hadn't been read and passed on to the front line clerks. So old tariffs continue to be used past their sell by date. (3) There were zealots who without a calculator, and probably working in poor light and a stuffy room, but clearly with a taste for mental arithmetic, managed to spot a deficiency and slap on Postage Due.