- The basic rules list "Find or Remove Traps" as a "double ability" but the expert rules only have "Remove Traps" in the table.
- The basic rules state "(thieves) are the only characters who can ... find traps without using magic..." on page B10 but "any character has a 1 in 6 chance of finding a trap when searching for one ... Any dwarf has a 2 in 6 chance. (This does not apply to magical traps..." on page B22.
Regarding the first point, note that neither the Greyhawk supplement for OD&D nor the Holmes edition have "Find Traps" as an option: they only list "Remove Traps" just like the expert set.
Regarding the second point, note that only one rule, either B10 or B22, makes sense. If we use B10 as the "correct" rule, than B22 must go away completely: Thieves only have 10% for "Find Traps" at first level and it would make thieves worse than everyone else at finding traps if B22 was also in play. If we use B22 as the "correct" rule, than B10 must go away completely: All characters get a chance to find traps, not only thieves.
I do have a theory about this, but that's really all it is: A theory. The 1981 basic and experts set seem to have been written/edited concurrently: Both list "January 1981" for their first printing and I've never heard that anyone had to "wait" for the expert set to be released (in other words, I am pretty sure they went on sale at the same time, not one after the other).
If that's true, and if they were primarily put together by the person given credit (so Tom Moldvay for the basic set and David Cook for the expert set), then it's also quite possible that each started working on "their" game independently using OD&D and Holmes as their sources. For some reason Moldvay (or Johnson or Mentzer, both of whom Moldvay thanks for "...their dedication in reorganizing and fine tuning this book.") eventually felt that thieves needed a "boost" to make them more important, and hence we get the rule on page B10. It wouldn't surprise me if that was a "last-minute" decision so neither the rule on page B22 nor the expert set was updated to be consistent with page B10.
As I said, it's just a theory. But it has enough explanatory power for me to "house-rule" that thieves in B/X do not in fact get the "Find Traps" ability at all. They get "Remove Traps" but the entire business of finding traps is handled with the "everybody has a shot" rule from B22. I think B/X is on the right track giving every character (regardless of class) a bunch of basic "dungeoneering" skills, and finding traps should definitely be included in that set.
Which leaves magical traps since they are explicitly ruled out on page B22. Personally I never liked the idea that thieves can do something that's clearly more in the domain of clerics or magic-users: spell-casters should be able to figure out magical traps, fighters and thieves shouldn't. True, in a fantasy world where many "worthwhile scores" are protected by magic, one could argue that thieves should develop such powers. However, it seems much more logical to say that they need to use "Wands of Trap Detection" to get this done. In fact, one could even use that to explain that high-level thieves get a "Use Magic Item" ability instead of a "Cast Spells from Scrolls" ability which again seems to encroach too much on clerics and magic-users.
And at least in my B/X game (I split up race and class) this makes the elusive elven magic-user/thieves and dwarven cleric/thieves that much more interesting.