system tablespace

I took over one of our oracle production databases (1.5 Tb) while the regular dba is out. I got errors (unable to extend temp segment) for two user tablespaces which I took care of by adding a datafile to each. While reviewing the other tablespaces, I was surprised to see the system tablespace at 99%. I checked with app folks. No new objects were added accidentally to it. I panicked and resized the system tablespace by an additional 33%. Upon review, I forgot to notice that the system tablespace was set to autoextend. My question is this: even with the autoextend option set, is there a performance hit or crash potential by not resizing the system (or any other) tablespace before it hits 100%? On the sybase side, I take care of the logical devices before it reaches 85%.