Startingstrength is very clear about the dangers of lifting to much weight. If your form starts to degrade, or the lift speed slows at the end of the 5 rep set, do not add anymore weight. Stay at that weight, or even decrease the weight until the complete set is done with no slow down and perfect form. If you blindly add five to ten pounds to your lift every session you will get injured, the book warns about this. Progressing slowly is not throwing out the whole program.