First-half goals from Noni Madueke and Viktor Gyokeres set the Gunners on course for victory in Czechia, but they needed to hold off a storm of Slavia pressure after the restart.
Wiktor Nowak netted from close range in the 90th minute, to jangle Arsenal's nerves even more, and Mikel Arteta admitted the performance was well below his expected standards.
A point on the road makes it 10 from four matches, as Arsenal sit just behind Real Madrid and PSG at the top of the table, and Arteta's men are now undefeated in their last 19 UCL games, their longest unbeaten streak in the competition.
Two other sides have gone 19 games without loss in Europe's top club competition - Bayern Munich and Ajax - and Arteta will hope to cruise past that mark and make it 20 at home to Borussia Dortmund at the end of November.
Only two sides have gone 20+, Manchester United (25 from 2007 to 2009) and Manchester City (26 from 2022 to 2024), and Arsenal will have both in their sights.