Toronto Blue Jays outfielder Lourdes Gurriel Jr. attempted a leaping catch against the Seattle Mariners that resulted in disaster instead of an out. The ball was in his glove perfectly before slipping out, and without his intereference it would not have gone over the fence.
This is a play that needs to be seen to be believed, so here it is, posted to Twitter by Daniel Kramer.

This is a once-in-a-season kind of play and Toronto Blue Jays fans could hardly grasp it. The play essentially gave the Seattle Mariners a free run.
The Blue Jays have been among the best in the MLB this season, but this game is not going their way. The Seattle Mariners would go on to extend their lead after this error.
This is a play that will live on forever in the history of the MLB, purely because of its absurdity.
The Seattle Mariners were gifted a home run by the Toronto Blue Jays in one of the craziest plays of the season

This is one of the strangest plays seen in a long time and it is almost without peer. It is, however, similar to a play from Jose Conseco while with the Texas Rangers, which many fans were quick to point out.
This could come as a blow to the confidence of Lourdes Gurriel Jr., who is a finalist for the upcoming All-Star game.
Smiling is about all the outfielder could have done in the aftermath of a play like that. Laughing at the ridiculousness of it is the only way to handle it. Hopefully it won't get him down, but if it does, fans are already brainstorming ways to cheer him up.
The Seattle Mariners fanbase is quite well adjusted to chaos and believes that a play like this is a good omen for the future.
Most Toronto Blue Jays fans were simply left stunned without explanation of how this happened. This is one of those rare moments where embracing the confusion is all that can be done.
The bizzare play gifted the Seattle Mariners a home run and left the Toronto Blue Jays scratching their heads. It's a moment that will not soon be forgotten and a gaff that will be replayed for years on end.