RoHS question

Almost all new equipment has to comply with "RoHS" (Reduction of Hazardous Substances) requirements, which among other things means it's made with lead-free solder. Enforcement is stronger in Europe than in the US, but in any case industry is cleary moving in that direction even when and where not absolutely required by law. Question: If one is repairing RoHS compliant equipment, with respect to functionality only is there a problem with using conventional tin/lead solder? I'm not asking if it's legal or "environmentally friendly", I'm asking if mixing the lead free solder and conventional tin/lead solder will cause functional problems (for example, any kind of problems similar to those which we used to see when someone repaired electronic equipment using plumbing acid flux solder). Also, if one wants to acquire and use RoHS compliant solder, are there any changes that someone accustomed to conventional tin/lead solder needs to make to their soldering technique? And I guess I should also ask the reverse question from the paragraph above: Are there any issues in using lead-free RoHS compliant solders on equipment originally built from tin/lead solder? Are there any other implications of RoHS that a casual classic computer and electronics enthusiast should know about, in either direction?