The AbstractSourceMachine#setSource method (and by extension, its addSource method) caps out the added source at the machine's maxSource value. However, the Sourcelink tile entities don't check for that, meaning they will continue to try and generate source even when it doesn't fit. In some cases, that can be excessively wasteful, just voiding source. For instance, a Volcanic Sourcelink with fuel from the Gobber mod can attempt to generate 50% source per item, leading to a constant waste of 40% per item (so an 80% loss overall) as it fills up.

Sourecelinks should check that the amount of source they're attempting to generate actually fits in their capacity, and bail if not. (Open for discussion: a compromise of a config option to say instead "stop consuming for generation if it would waste more than X%"?) Upgrading the fuel for source automation only to find that you're voiding most of the generation is a real feels-bad gameplay moment.