One thing I am wondering:

If you somehow manage to get a Strength of 26 (for example, by editing your save or the game itself, or by using something like Cheat Engine), what effects does that have?

A similar question could be asked for other stats, or if your character reaches a level beyond where the game's tables go.

The game cannot apply bonuses above and beyond those set out in the tables -- it knows nothing about such stats -- so you would get the bonuses for the 25 cap.

In the non-Enhanced editions I know some stats wrapped around. So it's very possible you would hit 0 and die.

Interestingly, this is not what happens in Pool of Radiance, where my third-gender character (hex-edited the gender field to "2", which is displayed as "COPPER" in-game) gets no bonus at all for Strength scores 26 and higher. (Male and female characters have different strength limits, and for whatever reason, a "copper" gender human cleric has a minimum Strength of 18 and a maximum Strength of 100, as can be seen by using the Modify Character option.)

In AD&D lore, as far as I remember, only the god Thor had strength 26 and that was because if his magic belt that increased it over the godly 25. Those with only 25 strength could only lift his hammer and those with 26 could use it as a weapon.

I think any stat over 18 is supposed to be supernatural. It's why humans can only be boosted to go past 18 and non-human races can have a natural 19+

It's the old one. The DOS version from gog.com.

(There's no guarantee that other versions, like the Commodore 64 version, behave the same way.)

Then it really shouldn't even register stats above 25. Even if the game did allow you to do beyond 25, it should have no additional effect. However, given that he result was out of the normal possible range, its probably that the engine just threw a fit.

It is getting on a bit, after all. The C64 version was basically identical to the DOS version, excepting in graphical capability.

The games may be almost identical from the standpoint of casual gameplay, but the actual contents of memory are different, as the graphics are different, and the CPU has a different instruction set (so the machine code is different). Therefore, if something depends on something like the memory layout or the contents of memory, the versions might behave differently.

To put it another way, once the game is forced to operate outside its normal parameters, version differences that are normally irrelevant start to matter.

(An interesting example is Super Mario World. There are a few glitches (cloud in item box, arbitrary code execution) that depend on Open Bus working the way it does on a Super NES; in the Virtual Console releases, Open Bus isn't emulated accurately, and hence such glitches generally don't work. Another example is Pokemon 1st gen: In the 3DS versions, one machine instruction is not implemented correctly, so dying when your money count contains digits outside the 0-9 range produces different results, breaking the 151 speedrun route.)