ipairs bugged?

[zgrssd]

As far as I understand the only difference between pairs and ipairs should be that ipairs goes by alphabetical/numerical order. Both should still go over all the data.

But when given a table like this:
[1] = "CHAT_CATEGORY_SAY"
[2] = "CHAT_CATEGORY_YELL"
[3] = "CHAT_CATEGORY_WHISPER_INCOMING"
[4] = "CHAT_CATEGORY_WHISPER_OUTGOING"
[6] = "CHAT_CATEGORY_ZONE"
... (the table is in total 41 entries long)

ipairs will stop dead after entry 4.
As opposed to pairs, wich goes properly over the full table.

As far as I understand the syntax those are global functions.
Any chance Zenimax bugged ipairs up?
Any chance some addon is overwriting ipairs with a faulty version?