Use a straight edge to continue the poorly cut piece's edge through onto the adjacent piece. The best way to do this is to put down a layer of making tape on the floor, and then a layer of making tape on a ruler or level.

Use glue (Elmer's, wood glue, hot glue, whatever, just NOT super glue or epoxy) on the tape to stick the straight edge down in position. Be really slow and careful to make sure it runs straight from the bad cut to the doorjam. Let it dry.

Then use a brand new box cutter to run along that edge and cut through the flooring piece. You'll probably need to snap the blade off every four or five passes. Go slowly so you don't walk the cut. Once the board is cut you can work a painter's scraper under the masking tape and get it off the floor clean, no mess. Now you have two straight edges on the gap that aren't square, but they're straight.

Then you make a template of the area with cardboard, because again it's not going to be square. Just tape a few pieces together to get your edges and things right.

Get yourself a transition from the big box store, or some other board that's a contrasting color. Whatever it is, it needs to be thinner than the surrounding floor by at least 1/16". Use your cardboard template to cut down your new board. If it's wood, buy yourself a little hand plane and use that to dial in toward the size of your template.

Then you use a thin layer of titebond adhesive on the floor and stick your new board down. Be gentle with pushing it down so that you make sure it goes even with your existing floor all the way around. Fill in perimeter gaps with black or grey caulk.

Then try not to look too closely at it ever again.