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The manual says:

The behavior of `next` is undefined if, during the traversal, you
assign any value to a non-existent field in the table. You may however
modify existing fields. In particular, you may clear existing fields.

Under 'pairs' it says:

See function `next` for the caveats of modifying the table during its traversal.

It does not say anything of the kind under `ipairs`, which does not depend
on `next`. The behaviour of operations like t[k]=nil, t[#t+1]=v,
table.insert(t,k,v), table.remove(t,k) is not undefined, but may require
some thought.

What does the following code do?

for k,v in ipairs(t) do if type(v)=='table' then
   if v[1] then
      table.insert(t,k,v[1]); table.remove(v,1)
   else table.remove(t,k)

Hint: it almost does what it should, but there is a bug.