A few months ago I have prepared several batches of samples (whole cell homogenates in Laemmli buffer) the exact same way every time. Every batch I would aliquot in volumes of 50 µl. One aliquot I used for an experiment right away, the others were frozen at -20 °C. They would all yield beautiful results on my gel.
Now, about 6 months later, after thawing, some of them work just as nice as the fresh samples, but some don't. Gels turn out completely ugly. They resemble samples with e.g. too much potassium in them, with lane narrowing and smear and all that. Though there's no potassium in them.
What else could be causing this? Is there anything I need to pay special attention to when thawing the samples?