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But it is 2.4 GHz!!!

Wireshark version 2.2.5.

It never caused me any problem, I am just curious why it is in this way. If it is just buggy AP firmware, wireshark wouldn't decode it.

Anyway, thank you for your time.

If the Information Element is present, it should be decoded correctly, even if it isn't expected.

But how do you decode correctly information element, id of which is "reserved"? You cannot determine by length which information element is it. Only by id. Which is 47 in this case, which is "reserved" by standard.

Obviously, they aren't obeying the standard.    Who is going to enforce it ?

The FCC has legal powers of arrest and seizure, but the IEEE does not.   I'm serious.

The FCC doesn't care if you follow protocol standards as  long as you meet THEIR regulations, which generally have nothing to do with frame protocols - they care about Layer 1 power levels, spectrum masks, adjacent channel interference, etc. 

If the mfg's device screws up, and isn't compatible with other 802.11 equipment - well in the long run only they will be hurt  (neglecting for the moment neighboring radios).

You would think the WFA would care about such things, but in fact they rarely do. 

At one time, if you used even a single bit in a frame that was "reserved", they would fail your certification test.    I know, because my company is a member and I've read the actual test specs (under a strict NDA).    Same goes for the UNH-IOL tests.

In many cases, using a reserved bit or string won't stop your device from working, it just may not work so well in the future - or  with someone else's non-standard device.

Face it. It happens.

If you want to see an area that is really bad when it comes to following the 802.11 standard, check the NONCE's generated by a large percentage of Wi-Fi gear and see how poorly they follow the cryptographic standards and controls that they are supposed to.

I think you don't understand my initial question. I am not thinking that IEEE can force someone to obey the standard. And deviations from standard are common.

My question is about the deviation itself. 

1. Some APs include tag with number 47 as ERP information.

2. Last version of wireshark recognizes tag with number 47 as "ERP information".

3. The name of this thread says that it could be 42 or 47.

If it would be just one vendor, which put some random ie into frame, i wouldn't even ask. Question raised from the fact that

1. Wireshark recognizes it as "ERP information".

2. I found several sites (including this one), which mention tag with number 47 as "ERP information"

Because of aforementioned reasons I believe that it is not just a coincidence, but a kind of "common practice". And this is what my initial question is about:

Who first started it? and for which purpose if tag 42 already exist? Why duplicated information element is needed?

Have you tried asking the people at Wireshark.org or Savvius (Wildpackets) ?

Either may be able to give you a better answer to this question.