Showalter was convinced that right-hander Ubaldo Jimenez tagged Jason Castro's left leg before he touched home plate following a wild pitch.
Showalter challenged the call and after a four-minute, 25-second review, the call stood, meaning there wasn't conclusive evidence to overturn it.
"He's out," Showalter said. "I'm not sure what they're looking at. I'll show you what we looked at. That's probably the third time that's happened. It's kind of frustrating. It puts umpires in a tough spot."
After the Orioles won their first two challanges this season, Thursday's was the third they've lost in their past five.
"I don't know how you get much more conclusive than what we saw," Showalter said. "But I don't know what they're looking at. They're supposed to have the same look we have. It didn't beat us."
Jimenez was uncharacteristically animated in disputing the call, pleading with home plate umpire Mike Muchlinski, even showing him a cleat mark on his glove.
"I'm pretty sure I had him because his cleat got caught on my glove," Jimenez said. "He almost made a hole in the glove. That's what I was trying to show the umpire. He never touched home plate. I have the hole mark from his cleat on my glove.
"I knew for sure he didn't touch home plate. It was my glove in front of home plate and his cleat got caught on it. It's a tough play to call and they didn't get it right, I guess."
Meanwhile, the Orioles are now 26-26, sitting at .500 for the first time since April 27, when they were 12-12. They've lost 12 of their past 18 games and the only saving grace is that most of the American League East – except for Toronto – seems to be sputtering at well.
And yes, Orioles fans, this team is become frustrating to watch, mainly because they've shown the ability to win. But when the pitching is there, the bats don't seem to be. And when the offense erupts, the Orioles end up involved in a slugfest.
"With the exception of a couple of guys, we're not swinging the bats real well," Showalter said. "But then one day we'll break out and score seven or eight runs. I feel better about the pitching tonight. The only good thing about losing it you don't have to pitch the ninth inning so hopefully we can get back on our feet. We've got some guys [in the bullpen] who needed a little blow, getting in late last night. It's been a challenge."
On Thursday night, Jimenez battled and left a 1-1 game after six innings. That should usually be good enough to get a win.
"I think the offense has been swinging the bat well lately as a whole," first baseman Chris Davis said. "We thought we were going to come in here tonight, continue that. We got shut down tonight. Ubaldo did throw the ball really well, that's what we need to see out of him."
And granted, this team has already overcome its share of adversity. Its full projected starting lineup hasn't played one game together, with Manny Machado, J.J. Hardy, Davis and now Matt Wieters all missing substantial time.
Through 52 games, the Orioles rotation has recorded just 18 quality starts, and just three in the club's last 11 games. The bullpen has been overworked and overused.