I personally find transitional coinage to be quite interesting. Why?
Before I start, I think I might need to define what transitional coinage is as some people might have different ideas of what it is.
I would define a transitional coinage as two different types of coins issued in the same year. Why this is interesting is because rather than waiting for the next design to be issued the next year, some urgent event may have prompted the need for the new coinage to appear faster than required. Some reasons are high metal prices, change of monarchy, inflation and so on. A good example is the 1982 penny where the composition was changed from copper to plated zinc in the same year rather than waiting it out to the next year.
Here are some examples that I have:
South Korea:
1970 5 and 10 won:
Red copper

Yellow copper

During the late 1960s, copper prices have shot drastically that the Korean mint was forced to reconsider the amount of copper that was used in their coinage. 1 won was soon switched from 60% Cu, 40% Zn to 100% in 1968. As if this wasn't enough, an urgent change was done in 1970 where copper content was dropped from 88% to 65%. You can tell that the color of coins are different.
This problem has reoccured later in 2006 where copper zinc coin still proved to be too expensive and it was replaced with copper plated aluminum coin.


Malaysia
In 1993, Malaysia decided to remove the dollar sign from the ringgit coin and replace it with the word "ringgit".


This caused a lot of confusion with the public as they believed the latter version is a counterfeited. Confusion coupled with circulating counterfeited ringgit coins would eventually make this coin demonitized in 2005.
China - Manchukuo
There's more examples of this but I decided to show just a couple.
In 1934, the reign has changed from Da Tong to Kang De


Critical shortage of war metals have caused copper, nickel and even aluminum to be systemically removed from circulation.
1944 Aluminum 5 fen

1944 Red fiber 5 fen

Would like to see what else you know or have.