Interesting that no one realizes this is actually a halacha question and depending on the situation the halacha differs…
visit http://www.businesshalacha.com/chaburos/interferance-ani-mihapech-bicharara
Rav Nochum Sauer is the Rosh Chabura for the LA weekly chabura and he recently covered this sugya.
and here’s is a brief synopsis though not guranteed for accuracy
Standing on line:
Does a sick person at end of line have a right to go to the front?
What if the person at end of line has only 1 small quick item to buy, does he have to wait for all the other customers with much bigger loads?
What if the person at end of line will miss tefilah btzibur, or he’ll miss his carpool, can he move to the front?
First come, first serve: What is the halachic basis of this and what are the exceptions?
Sanhedrin 32: tzedek tzedek sometimes din, sometimes pshara compromise.
If two boats can’t pass at the same time or two camels can’t pass a narrow area at the same time then which boat or camel can go first?
If one is loaded with a lot of merchandise, and it needs to get to port to sell it, or there will be a hefsed, it has the right to go first. That is the tzedek, right and fair.
Or if one is closer to its city the one with the longer distance to go should go first.
If they are both equal, both loaded, both same distance, then make a pshara one lets the other go, but the other has to compensate the one that has to wait.
Meiri (Sanhedrin 32) defines the parameters. It depends on whats just and whats equitable. Whoever has less hardship is pushed aside, and so too a healthy person is pushed aside for a person who is sick. So too, for a bais din, there is an order yasom, almana, talmid chacham before am ha-aretz, and woman before a man to minimize the embarrassment.
There is a concept of zechus Ha-tor. If all else is equal, then makdimim ha-kodem first come first serve also based on tzedek tzedek tirdof. In a doctors office, if a person is very sick, he should come first, even when its not really pikuach nefesh.
Similarly, at the airport, we should let someone at back of the line go first so that they dont miss their plane.
What is the issur of cutting in front of someone in line? —
Possibly tzedek tzedek tirdof like Meiri.
Rav Elyashiev (hizaharu bmamon chavreichem) its avak gezel (shades of gezel). Possibly he means gezel zman.
Others say its like ani hamehapech you are taking away a certain opportunity that the first person has (he had the opportunity to finish his task sooner).
This is not something thats aino matzui like hefker or metzia, so even acc to R Tam ani hamehapech would apply.
Pischei Choshen (Rav Bloy?) ch 9 : cutting in line is not an issur of ani hamehapech you are not being kone an object. You are only causing a loss of zman, — there is no cheftza. He thinks the issur is based on minhag hamedina.
Mishpetei Torah (r Shpitz) (siman 84): it could be an issur of lo sonu ish es achiv. You are causing anguish to someone else. Also, there is potential chilul Hashem and machlokes.
R Zalman Nechemia Goldberg (quoted in Halachos of other peoples money R Bodner, Feldheim publishers) its a gezel from the owner of the store. The owner does not want people to cut in line which will cause chaos and customers will not come back so not following the rules of the baal bayis is like trespassing which is an issur gezel.
Cutting to the front of line:
You have to ask reshus of everyone on line, not just the first person.
Steipler on the bus:
He was standing at the back of the line, and he said he would only go to the front if everyone agreed, and he said he would be a gazlan if he went to the front without reshus. A bus might be more chamur than other cases since there are a limited number of seats it might be close to gezel. (He did not want to rely on his being a talmid chacham).
If someone is standing in line, can you hand a few items to him to buy for you rather than you having to wait in the long line yourself?
What if you give him the items before he gets in line?
Mishpetei Torah: you are not allowed to give the person in line extra things to buy for you. He was standing in line for himself. The tzedek and the yosher is that he buys only his own stuff. Before he gets on the line then its ok to give him a few extra things to buy. He could have bought more stuff for himself, so he can add things for someone else.
R Shpitz says that even if you are giving the stuff to him before he gets on line there is a limit you cant give him more that what a person would normally reasonably buy for himself.
If you are in line, and you forgot something, you dont lose your space, unless you are leaving for a long time.
If you are leaving for a long time, you lose your spot in the line.