Possible causes are hairline cracks in the front part, a defective sac converter, damaged seals, an operating error or too deep a grip in which the fingers touch the nib corners. Iron-gall ink should be flushed out immediately, since it is unsuitable for many fountain pens and on drying can destroy the feed; in such cases only an exchange of the feed helps. Since with other writing instruments no problem occurs, grip errors are unlikely; the front part should be checked for visible hairline cracks.