Yep, looks like a removeable snout to the liner tee that got misaligned.
You mentioned something about a 90% furnace. I hope it is not vented into this liner as that is a major safety issue. You cannot vent positive flue gas pressure combustion appliances with any other appliance much less an atmospherically vented one. Also, there are only two liners listed for positive flue gas pressure as most are Not approved for such use.
Dude, sorry but your terminology is killing me! Please forgive me while I make a few corrections:
The 'chimney' is the vertical structure. Inside the chimney is a flue, which should be lined but isn't always and most terra cotta liners fail requiring 'relining'. You may also need to reline for sizing. Now the pipe from the appliance to the flue liner (whether terra cotta, stainless steel, aluminum or poured cementitious liner) is called the 'connector'. For oil, it is referred to as the 'chimney connector' while gas appliances use a 'vent connector'. The vent connector in the case of a liner with a tee would include the snout of the tee, which is ajar in the OP's photo and not properly engaged into the tee body. Properly done, the male crimped end would have a full even reveal inside the tee and the stainless steel band clamp drawn tight pulling it into the tee. The 'flue' is the liner itself in this case.
Glad the 90%'er is not common vented into this liner. Whew!
BTW, 'linear' means in a straight line as opposed to wandering or abstract.