The Queen's speech is not a confidence vote. Neither the main vote at the end of the debate nor any amendments will automatically trigger the fall of the government if it loses a vote.
If the government were to lose the main vote on the Queen's speech, then there would almost certainly be a no confidence motion tabled. If Labour passes an amendment, then the PM must resign It would then be up to the opposition party to decide if they want to table a motion of no confidence.
There is the possibility of the Queen's speech not being passed, but a no-confidence motion also not passing. This would result in the odd situation of a government without a legislative programme. Again the result would almost certainly be the resignation of the PM and perhaps the senior members of the cabinet.
In May 2016 there was the possibility that rebel Tory MPs might have been able to get an amemdment opposing TTIP into the Queen's speech. This was seen as potentially "humiliating", but not a confidence matter.
In other circumstances, the government may accept an opposition amendment, if it is consistent with their overall stance, and has cross-party support. Better to accept than risk a show down. Again, this would not result in the government folding.
There was the odd sitution proposed in the New statesman of the Prime minister of a coalition government supporting an amendment to the Queen's speech, against the wishes of his coalition partner. Again the consequences of this were not tested.
These situations remain hypothetical, as they would require either a significant Tory rebellion on the Queen's speech, or the DUP reneging on an agreement they made this week - neither of which is remotely plausible.