In one of the flashbacks in SAW 4 just after Jill interrogated John about Cecil we get a short look at the glass coffin which ultimately killed Strahm in SAW 5. However I think this wasn't its actual purpose for some reasons:
1. It is clearly Hoffman's voice on the last tape for Strahm in the coffin room. If John already planned that Strahm would be involved in this test he would've recorded the tape on his own.
2. At the time of this flashback he probably didn't even know about Strahm and Perez. It's even likely that these two weren't even involved in the case yet.
According to the audio commentary of SAW 4 the coffin was originally planned to be used to test the aggressive vagrant who yelled at Rigg at the Alexander Motel. Unfortunately this scene was cut out probably due to being too brutal. If the vagrant was tested it must've happened at some point after SAW 4 and then noone ever mentioned it in any of the sequels. However I think it is more likely that the coffin was used for a Jigsaw victim between Cecil and Seth. We know there must've been one as Hoffman used a Billy puppet for Seth's tape even though this victim wasn't mentioned on the list in SAW 5. What do you guys think? Was the coffin used on the vagrant at some point after SAW 4 or do you agree with me that is was likely used to test another person we don't know about yet? And for whatever it was used for, how likely do you think it is that we'll find out about it in any future SAW movie?