There a couple of incidents involved, one of which happened previously, and another of which is occurring now.
The first incident
The first incident happened when my Life Cleric (level 6-7) found a magical sword (Sentient Sunblade). My character did not have the proficiency to use a longsword and we had a bunch of characters that do have the proficiency (a Bard, a Paladin, a Warrior, and a Ranger). Still, I felt that my character had a claim, as he did the "heavy lifting" on acquiring the sword (turning and defeating the ghost guarding the sword).
The problem was when the group discussed the distribution, the following occurred:
First, the session came to a halt over the discussion of who gets the sword. All 4 sword users discussed who should get the sword and the DM washed his hands, to the point of declaring the sentient sword had no preference over who got to use it. To solve the impasse, I took my claim for the sword and declared I would lend the sword per session on a rotation until my character got could use it properly (and I planned on learning the proficiency via feat - which I did end up taking), so I roleplayed this as my character reluctantly lending the sword each session to the other sword users.
The next session, one player (lets call her Vanessa, which is obviously not her real name but a pseudonym for the sake of clarity) seemed to get into her head that she is the Master Looter, so she declared that her character stole the sword from my cleric when he was "asking for spells" (read: I was setting up my spell list before the start of the game), no rolls or nothing, and gave it away to the character Vanessa felt deserved it (this third player later gave me back the sword after I became upset).
This... made me react a bit more strongly than the situation deserved perhaps; I was angry that Vanessa would force her will upon my character and decided my cleric was pissed and refused to follow the party to a cavern (under the excuse of guarding NPCs).
Once they came back Vanessa said her character would slap my cleric until I told Vanessa that my cleric would retaliate, so Vanessa took it back and only said that her character was disappointed, my comeback was that my cleric did not care about her disappointment (I feel I was being petty at that moment but it really angered me that she forced her way of doing things upon my character).
After the game, I had a bit of a discussion with the GM and he told me the Vanessa would be punished, though he was not clear in which way. The next session Vanessa's character got an alignment penalty (from LG to TN) because she found a cursed amulet that caused a madness roll that she failed (so removable with a restoration spell), though Vanessa seems to resent her punishment, I can tell because of some snide comments when my character gets trounced in combat.
It's been months since this happened. After the scene, the sword users were free to choose a reward and got weapons equal to or better than the sword, so it's not a current issue.
The second incident
Now the same series of events seems to be likely to repeat itself.
I managed to beat an archmage at a betting game with a shield guardian at stake, everyone got a chance to play and I won it in the second round, thanks to a good roll. Despite my character winning and therefore having a valid claim, Vanessa's first and so far only comment when discussing who should have it was "not the cleric, of course", on a Facebook group.
I do not particularly care much for the shield guardian and would give it up if someone asked, but I do not want to back down when Vanessa wants to force her will upon my character, especially when it is my character the one that finds/wins these magical items, but I don't want to start drama either.
Generally in our group, we give the loot to whoever is better able to use it, otherwise is finders-keepers unless he/she wants to give up the loot. The sword was a special case because there was a 5-way argument between all of our potential sword users over who got it.