All the talk in the comments about dissolved oxygen brings back a few memories of student lab work that went wrong. I was given a freeze-pump-thaw protocol to follow in order to remove oxygen from a copolymer solution. Basically we used liquid nitrogen to freeze the mixture, then used a vacuum pump to remove the gases above the mixture. When we thawed the mixture, the dissolved oxygen would escape, in order to equilibrate with the amount above the surface. I repeated this for a few cycles. It was all done with sealed glassware.

Unfortunately, the glassware kept cracking during the thaw stage. This went on for a couple of months, till finally my supervisor reviewed the procedure, and realised that, whether cracking occurred, depended upon the relative concentrations of the two polymers. If I recall correctly, the problem occurred when the polymer with the lower freezing point (which, thereby, thawed first) was present at a lower concentration. It was basically trapped inside the other still-frozen polymer, and expanding, as it thawed ... till crack!

The solution was surprisingly simple. Change the method. Bubble nitrogen through the mixture to drive out (sparge) the oxygen. Replacing the oxygen with nitrogen was beneficial, as nitrogen didn't interfere with the copolymerisation reaction.

The main problem was that I had lost two months of experimental time, and this was just the start of a series of experiments and instrumental analyses. It was now Easter and the deadline to hand in my student thesis was early December.

Please don't believe anyone who says students have an easy life.