One type of unit, with two levels of technology bug

can happen when units are traded

known exploit, since the SBDE applies to each of the split levels individually.

Haven't been traded in this case

It can happen when you capture a province when it's recruiting a unit. The unit will have the level of the player that owned the province before you.

Petouche wrote:

It can happen when you capture a province when it's recruiting a unit. The unit will have the level of the player that owned the province before you.

He is correct. For instance, if the province is building a Battleship, and you do not have it researched yet, it will continue

production and then you cannot build any additional at that location until you research it.

Petouche wrote:

It can happen when you capture a province when it's recruiting a unit. The unit will have the level of the player that owned the province before you.

Glad to find the solution

Regardless of how the odd-level unit was acquired, the lower level unit should update to your current level of research (and conform to the same level as all of your other units) when you complete the next research level for that particular unit type.